Medicine definitions

244 medicine terms from Achievable exam prep courses, each defined in plain language with links into the course textbook.

Acute coronary syndrome (ACS)Acute coronary syndrome (ACS) is a spectrum of conditions caused by a sudden reduction in blood flow to the heart muscle, encompassing unstable angina, NSTEMI, and STEMI. It most often results from rupture of an atherosclerotic plaque with thrombus formation.
AdenovirusAdenovirus is a non-enveloped, double-stranded DNA virus that commonly causes respiratory infections, conjunctivitis, and gastroenteritis. Outbreaks classically occur in crowded settings such as military barracks and swimming pools.
Adipose tissueAdipose tissue is specialized connective tissue composed of fat cells (adipocytes) that stores energy as triglycerides, insulates and cushions the body, and functions as an endocrine organ by secreting hormones such as leptin.
Adrenal glandsThe adrenal glands are paired endocrine glands sitting atop each kidney. The outer cortex secretes aldosterone, cortisol, and androgens, while the inner medulla secretes the catecholamines epinephrine and norepinephrine.
Adrenal insufficiencyAdrenal insufficiency is a condition in which the adrenal glands produce too little cortisol, and sometimes too little aldosterone. It can be primary (adrenal gland failure, called Addison disease) or secondary to inadequate ACTH from the pituitary.
AnemiaAnemia is a reduction in red blood cells or hemoglobin that lowers the blood's oxygen-carrying capacity. It is classified by red cell size (MCV) into microcytic, normocytic, and macrocytic types, each with distinct causes.
Antibiotic resistanceAntibiotic resistance is the ability of bacteria to survive drugs that would normally kill them or stop their growth. Resistance arises through mutation and the transfer of resistance genes, making infections harder to treat.
Antigen processingAntigen processing is the pathway by which cells break down proteins into peptide fragments and load them onto MHC molecules for display to T cells. Intracellular antigens are presented on MHC class I to CD8+ T cells, while extracellular antigens are presented on MHC class II to CD4+ T cells.
ArrhythmiaAn arrhythmia is any abnormality in the heart's rate or rhythm, caused by problems with the electrical impulses that coordinate heartbeats. Arrhythmias range from benign premature beats to life-threatening ventricular fibrillation.
Arterial blood gasesArterial blood gases (ABGs) are laboratory measurements of pH, oxygen (PaO2), carbon dioxide (PaCO2), and bicarbonate in arterial blood, used to assess oxygenation, ventilation, and acid-base status.
Ascertainment biasAscertainment bias is a systematic error that occurs when a study sample is not representative of the target population because of how subjects were identified or selected, distorting the study's results.
Atherosclerosis vs. arteriosclerosisArteriosclerosis is the general term for hardening and loss of elasticity of arterial walls, while atherosclerosis is its most common subtype, caused by lipid-rich plaques building up in the arterial intima.
AuscultationAuscultation is the physical examination technique of listening to internal body sounds, usually with a stethoscope. Clinicians auscultate the heart, lungs, and abdomen to assess valve function, breath sounds, and bowel activity.
AutoclaveAn autoclave is a device that sterilizes instruments and supplies using pressurized steam, typically at 121°C (250°F) for a set exposure time. The combination of heat, moisture, and pressure destroys all microorganisms, including bacterial spores.
Bacillus anthracisBacillus anthracis is the gram-positive, spore-forming, rod-shaped bacterium that causes anthrax. It is known for its unique protein capsule and a three-part exotoxin, and it produces cutaneous, inhalational, and gastrointestinal disease.
Bacillus cereusBacillus cereus is a gram-positive, spore-forming rod that causes food poisoning. Its heat-stable emetic toxin, classically linked to reheated rice, causes rapid vomiting, while a separate enterotoxin causes a later-onset diarrheal illness.
Basal ganglia pathwaysThe basal ganglia pathways are the direct and indirect circuits through which deep brain nuclei regulate movement. The direct pathway facilitates intended movement, while the indirect pathway suppresses unwanted movement.
Blood cell lineageBlood cell lineage describes how all blood cells descend from a single hematopoietic stem cell in the bone marrow, branching into the myeloid line (red cells, platelets, granulocytes, monocytes) and the lymphoid line (B, T, and NK cells).
Blood transfusion reactionsBlood transfusion reactions are adverse immune or non-immune responses to transfused blood products, ranging from mild fever and hives to acute hemolysis, anaphylaxis, and transfusion-related acute lung injury (TRALI).
Body fluid compartmentsBody fluid compartments are the divisions of total body water: intracellular fluid inside cells and extracellular fluid outside them, with the extracellular portion split between interstitial fluid and plasma.
Bordetella pertussisBordetella pertussis is a gram-negative coccobacillus that causes pertussis, or whooping cough. It attaches to respiratory epithelium and releases toxins that produce a prolonged, paroxysmal cough illness.
Borrelia burgdorferiBorrelia burgdorferi is the spirochete bacterium that causes Lyme disease, transmitted to humans through the bite of infected Ixodes (deer) ticks. Its hallmark early sign is erythema migrans, the expanding bull's-eye rash.
Calcium homeostasisCalcium homeostasis is the set of hormonal mechanisms that hold serum calcium within a narrow range by adjusting absorption from the gut, reabsorption in the kidney, and exchange with bone. Parathyroid hormone, calcitriol, and calcitonin are the three principal regulators.
Campylobacter jejuniCampylobacter jejuni is a curved, comma-shaped gram-negative bacterium and one of the most common causes of bacterial gastroenteritis worldwide. It is transmitted mainly through undercooked poultry and unpasteurized milk.
Capillary punctureA capillary puncture is a blood collection method that uses a lancet to pierce the skin and obtain a small sample of capillary blood. It is used when only a small volume is needed or when venipuncture is impractical, as in infants.
Cardiac action potentialThe cardiac action potential is the sequence of ion movements that depolarizes and repolarizes a heart cell, triggering contraction. Working myocytes follow a five-phase pattern with a calcium plateau, while pacemaker cells depolarize spontaneously.
Cardiac cycleThe cardiac cycle is the sequence of electrical and mechanical events that occurs with every heartbeat. It divides into systole, when the ventricles contract and eject blood, and diastole, when they relax and fill.
Cardiac hypertrophyCardiac hypertrophy is an increase in heart muscle mass caused by enlargement of individual cardiac myocytes. It develops as an adaptation to chronic pressure or volume overload and, when pathologic, predisposes to heart failure and arrhythmia.
Cardiac rehabilitationCardiac rehabilitation is a medically supervised program of graded exercise, risk-factor education, and counseling for patients recovering from a cardiac event or procedure. It progresses through inpatient, outpatient monitored, and community maintenance phases.
Cardiac stress testA cardiac stress test measures how the heart responds when its workload is increased, usually by walking on a treadmill while an ECG and blood pressure are monitored continuously. It is used to detect reduced blood flow to the heart muscle that may not appear at rest.
CardiomyopathyCardiomyopathy is disease of the heart muscle itself that impairs its ability to fill or eject blood. The three principal structural categories are dilated, hypertrophic, and restrictive cardiomyopathy.
Cerebellum and brainstemThe cerebellum is the hindbrain structure that coordinates movement, balance, and motor learning, while the brainstem — midbrain, pons, and medulla — relays signals between the brain and spinal cord and controls vital autonomic functions such as breathing and heart rate.
Cerebral blood supplyThe cerebral blood supply is the arterial network that perfuses the brain, fed by the paired internal carotid arteries anteriorly and the vertebral arteries posteriorly. These systems join at the circle of Willis, which distributes blood through the anterior, middle, and posterior cerebral arteries.
Chlamydia and ChlamydophilaChlamydia and Chlamydophila are genera of obligate intracellular bacteria that cannot make their own ATP and must replicate inside host cells. They cause genital and ocular infections, atypical pneumonia, and psittacosis.
CholangiocarcinomaCholangiocarcinoma is a malignant tumor arising from the epithelial cells that line the bile ducts. It is usually an adenocarcinoma, tends to present late with painless jaundice, and carries a poor prognosis because it is rarely resectable at diagnosis.
CholangitisCholangitis is inflammation of the bile ducts, most often caused by bacterial infection behind a blocked duct. Acute ascending cholangitis is a surgical emergency marked by right upper quadrant pain, fever, and jaundice.
Chromosomal disordersChromosomal disorders are conditions caused by a missing, extra, or structurally altered chromosome rather than a mutation in a single gene. Because each chromosome carries hundreds of genes, these disorders usually affect multiple organ systems at once.
CirrhosisCirrhosis is end-stage chronic liver disease in which normal liver tissue is replaced by fibrous scar tissue and regenerative nodules. It impairs liver function and obstructs blood flow, causing portal hypertension and its complications.
Clostridium botulinumClostridium botulinum is a gram-positive, spore-forming anaerobic bacterium that produces botulinum toxin, the cause of botulism. The toxin blocks acetylcholine release at the neuromuscular junction, producing a descending flaccid paralysis.
Clostridium difficileClostridium difficile (now Clostridioides difficile, or C. diff) is a spore-forming, gram-positive anaerobic bacterium that causes antibiotic-associated diarrhea and pseudomembranous colitis, most often after broad-spectrum antibiotics disrupt normal gut flora.
Clostridium perfringensClostridium perfringens is a gram-positive, spore-forming, anaerobic rod that causes gas gangrene (clostridial myonecrosis) and a common self-limited food poisoning. Its key virulence factor is alpha toxin, a lecithinase.
Clostridium tetaniClostridium tetani is a gram-positive, spore-forming anaerobic bacillus that causes tetanus. Its toxin, tetanospasmin, blocks inhibitory neurotransmitter release, producing sustained muscle rigidity and spasms such as lockjaw.
Coagulation cascadeThe coagulation cascade is the sequence of enzyme activations that converts circulating clotting factors into a stable fibrin clot. Its intrinsic and extrinsic pathways converge on a common pathway that activates thrombin and forms fibrin.
Confidence interval vs. standard deviationStandard deviation measures how spread out individual data points are around a sample's mean, while a confidence interval expresses how precisely a sample statistic estimates the true population value. Standard deviation describes variability; a confidence interval describes estimation uncertainty.
COPD (chronic obstructive pulmonary disease)COPD is a progressive obstructive lung disease characterized by persistent airflow limitation that is not fully reversible. It encompasses chronic bronchitis and emphysema, and is most often caused by long-term cigarette smoking.
Corynebacterium diphtheriaeCorynebacterium diphtheriae is the gram-positive, club-shaped bacillus that causes diphtheria. Its exotoxin inhibits protein synthesis by ADP-ribosylating elongation factor 2, producing the characteristic gray pharyngeal pseudomembrane.
Coxiella burnetiiCoxiella burnetii is an obligate intracellular bacterium that causes Q fever, an infection acquired by inhaling aerosols from infected livestock. It classically presents as an atypical pneumonia or hepatitis without a rash.
CoxsackievirusCoxsackievirus is an enterovirus in the Picornaviridae family, divided into groups A and B. Group A classically causes hand-foot-and-mouth disease and herpangina, while group B is associated with myocarditis, pericarditis, and pleurodynia.
Cranial nervesThe cranial nerves are twelve pairs of nerves that arise directly from the brain and brainstem rather than the spinal cord. They carry sensory and motor signals for the head and neck, including smell, vision, facial movement, hearing, and swallowing.
CryptosporidiumCryptosporidium is a protozoan parasite that infects the small intestine and causes watery diarrhea. It spreads through contaminated water and resists chlorine, making it a frequent cause of waterborne and swimming pool outbreaks.
Cushing's syndromeCushing's syndrome is the clinical condition caused by chronic exposure to excess cortisol. Hallmark features include central obesity, moon facies, a dorsal fat pad, purple striae, hypertension, and hyperglycemia.
Cycle of infectionThe cycle of infection is the six-link sequence a pathogen must complete to spread disease: infectious agent, reservoir, portal of exit, mode of transmission, portal of entry, and susceptible host. Breaking any link stops the infection from spreading.
DementiaDementia is a progressive decline in cognitive function — memory, language, judgment, and reasoning — severe enough to interfere with daily life. Alzheimer disease is its most common cause, followed by vascular dementia.
Developmental milestonesDevelopmental milestones are skills most children achieve by predictable ages — such as sitting, walking, and first words — used by clinicians to track normal development and catch delays early.
Diabetes mellitusDiabetes mellitus is a group of metabolic disorders characterized by chronic hyperglycemia resulting from defects in insulin secretion, insulin action, or both. The two main forms are type 1 and type 2.
Disorders of amino acid metabolismDisorders of amino acid metabolism are inherited enzyme deficiencies that block the breakdown or conversion of specific amino acids, causing toxic metabolites to accumulate. Classic examples include phenylketonuria, maple syrup urine disease, and homocystinuria.
DuodenumThe duodenum is the first and shortest segment of the small intestine, a C-shaped tube about 25 cm long that receives chyme from the stomach along with bile and pancreatic secretions, beginning the major phase of chemical digestion.
Electrocardiography (ECG)Electrocardiography is the recording of the heart's electrical activity through electrodes placed on the skin. The resulting tracing, an electrocardiogram (ECG or EKG), displays the heart's rhythm and conduction as a series of characteristic waves.
Electrolyte imbalancesElectrolyte imbalances are abnormally high or low blood levels of minerals such as sodium, potassium, calcium, and magnesium. Because these ions govern nerve conduction, muscle contraction, and fluid balance, imbalances can cause symptoms ranging from weakness and confusion to fatal cardiac arrhythmias.
End-systolic volumeEnd-systolic volume (ESV) is the amount of blood remaining in a ventricle at the end of contraction, roughly 50 mL in a healthy adult left ventricle. It is the low point of ventricular volume and a key input to stroke volume and ejection fraction.
Entamoeba histolyticaEntamoeba histolytica is a protozoan parasite that causes amebiasis — bloody diarrhea (amebic dysentery), colonic flask-shaped ulcers, and liver abscesses. It spreads by fecal-oral transmission of cysts, often via contaminated food or water.
Enterobius vermicularisEnterobius vermicularis, commonly called pinworm, is an intestinal nematode that causes nocturnal perianal itching, most often in children. It is the most common helminth infection in the United States and is diagnosed with the tape test.
EnterococciEnterococci are gram-positive cocci that normally inhabit the human gastrointestinal tract. Enterococcus faecalis and Enterococcus faecium are opportunistic pathogens known for causing urinary tract, biliary, and bloodstream infections and for resisting many antibiotics.
Enzymes and substratesAn enzyme is a biological catalyst, almost always a protein, that speeds a reaction by lowering its activation energy without being consumed. The substrate is the molecule the enzyme acts on, binding to a specific pocket called the active site.
ErythropoietinErythropoietin (EPO) is a hormone produced mainly by the kidneys that stimulates the bone marrow to make red blood cells. Its release is triggered by low oxygen delivery to the kidney, making it the body's central regulator of red cell mass.
Escherichia coli (E. coli)Escherichia coli is a gram-negative, rod-shaped bacterium that normally inhabits the human intestine. Most strains are harmless, but pathogenic strains cause urinary tract infections, traveler's diarrhea, neonatal meningitis, and hemolytic uremic syndrome.
Esophageal diverticulaEsophageal diverticula are outpouchings of the esophageal wall. They are classified by location as pharyngoesophageal (Zenker), midesophageal (traction), or epiphrenic, and by whether they involve all layers of the wall.
Fallopian tubesThe fallopian tubes are a pair of muscular tubes that carry an oocyte from the ovary to the uterus. Fertilization normally occurs inside them, and they transport the resulting embryo to the uterine cavity for implantation.
Fetal circulationFetal circulation is the pattern of blood flow before birth, in which the placenta rather than the lungs performs gas exchange. Three shunts — the ductus venosus, foramen ovale, and ductus arteriosus — divert blood away from the liver and lungs.
FeverA fever is a regulated rise in body temperature caused by an upward shift in the hypothalamic set point, generally defined as a temperature of 100.4°F (38.0°C) or higher. It is a common sign of infection and other inflammatory processes.
Fifth left intercostal spaceThe fifth left intercostal space is the gap between the fifth and sixth ribs on the left side of the chest. At the midclavicular line, it marks the apex of the heart, where the apical impulse is palpated and the mitral valve is auscultated.
Fluid and electrolyte disordersFluid and electrolyte disorders are conditions in which body water volume or the concentration of key electrolytes — sodium, potassium, calcium, magnesium, phosphate — falls outside its normal range. They arise from losses, excess intake, or impaired regulation by the kidneys and endocrine system.
GaitGait is the pattern of movement used in walking, described as a repeating gait cycle from one heel strike to the next heel strike of the same foot. Each cycle divides into a stance phase, when the foot is on the ground, and a swing phase, when it is not.
GeneA gene is a segment of DNA that carries the instructions for making a functional product, usually a protein or an RNA molecule. Genes are the basic units of heredity, passed from parents to offspring.
Genetic codeThe genetic code is the set of rules that translates nucleotide triplets in mRNA into amino acids in a protein. Its 64 codons specify 20 amino acids plus start and stop signals, and it is read three bases at a time without overlap.
Giardia lambliaGiardia lamblia is a flagellated protozoan parasite that colonizes the small intestine and causes giardiasis. Infection produces greasy, foul-smelling diarrhea and is classically acquired by drinking untreated stream or well water.
Glomerular filtrationGlomerular filtration is the first step of urine formation, in which pressure in the glomerular capillaries drives water and small solutes out of the blood and into Bowman's capsule. The rate of this process, the GFR, is the standard measure of kidney function.
Glycogen metabolismGlycogen metabolism is the set of pathways that build and break down glycogen, the branched storage form of glucose found mainly in liver and skeletal muscle. Glycogenesis stores excess glucose after a meal, and glycogenolysis releases it during fasting or exercise.
GlycolysisGlycolysis is the metabolic pathway that breaks one molecule of glucose into two molecules of pyruvate in the cytoplasm. It yields a net gain of 2 ATP and 2 NADH and requires no oxygen.
Gram-negative cocciGram-negative cocci are round bacteria that stain pink because their thin peptidoglycan layer does not retain crystal violet. The clinically important human pathogens in this group are Neisseria meningitidis, Neisseria gonorrhoeae, and Moraxella catarrhalis.
Haemophilus influenzaeHaemophilus influenzae is a small gram-negative coccobacillus that causes respiratory infections, otitis media, epiglottitis, and meningitis. Despite the name, it is a bacterium and does not cause influenza.
Heart failureHeart failure is a clinical syndrome in which the heart cannot pump enough blood to meet the body's metabolic demands, or can do so only at elevated filling pressures. It produces congestion behind the failing chamber and reduced perfusion ahead of it.
Helicobacter pyloriHelicobacter pylori is a curved, gram-negative bacterium that colonizes the lining of the stomach. It is the leading cause of chronic gastritis and peptic ulcer disease and is a recognized risk factor for gastric cancer.
Hemodynamic monitoringHemodynamic monitoring is the continuous or serial measurement of blood pressure, blood flow, and oxygenation to assess cardiovascular function in critically ill patients. It ranges from noninvasive vital signs to invasive arterial and pulmonary artery catheters.
Hemolytic anemiaHemolytic anemia is anemia caused by the premature destruction of red blood cells, either within blood vessels (intravascular) or by the spleen and liver (extravascular). Hallmarks include elevated LDH, low haptoglobin, and reticulocytosis.
Hemophilia A vs. von Willebrand diseaseHemophilia A is an X-linked recessive deficiency of coagulation factor VIII that causes deep bleeding into joints and muscles, while von Willebrand disease is a usually autosomal dominant defect of von Willebrand factor that impairs platelet adhesion and causes mucocutaneous bleeding.
Hepatitis virusesThe hepatitis viruses — hepatitis A, B, C, D, and E — are five unrelated viruses that all target the liver. They differ in transmission route, chronicity, and prevention: A and E spread fecal-orally, while B, C, and D spread through blood and body fluids.
Hereditary spherocytosisHereditary spherocytosis is an inherited red blood cell membrane disorder in which defective structural proteins cause red cells to become sphere-shaped. The spleen destroys these fragile spherocytes, producing a chronic extravascular hemolytic anemia.
HerpesvirusesHerpesviruses are a family of enveloped, double-stranded DNA viruses that establish lifelong latent infections in humans. Members include HSV-1, HSV-2, varicella-zoster virus, Epstein-Barr virus, and cytomegalovirus.
Hip range of motion normsHip range of motion norms are the reference values for normal movement at the hip joint, commonly cited as about 120° of flexion, 20° of extension, 40–45° of abduction, 20–30° of adduction, and roughly 45° of internal and external rotation.
Hodgkin vs. non-Hodgkin lymphomaBoth are cancers of lymphocytes, but Hodgkin lymphoma is defined by the presence of Reed-Sternberg cells and spreads in an orderly, contiguous fashion between adjacent lymph node groups. Non-Hodgkin lymphoma lacks Reed-Sternberg cells and spreads unpredictably, often involving extranodal sites.
HookwormsHookworms are parasitic intestinal nematodes — chiefly Ancylostoma duodenale and Necator americanus — whose larvae penetrate the skin, migrate to the small intestine, and feed on blood, causing iron deficiency anemia.
Hymenolepis nanaHymenolepis nana, the dwarf tapeworm, is the smallest and most common tapeworm infecting humans. It spreads by fecal-oral ingestion of eggs, can complete its entire life cycle in one host, and is treated with praziquantel.
Hypersensitivity reactionsHypersensitivity reactions are exaggerated or inappropriate immune responses that damage the host. The Gell and Coombs system classifies them into four types: I (IgE-mediated), II (antibody-mediated cytotoxic), III (immune complex), and IV (T-cell-mediated delayed).
HypertensionHypertension is chronically elevated arterial blood pressure. Usually symptomless, it is a major risk factor for heart attack, stroke, heart failure, and kidney disease, which is why it is called the "silent killer."
HyperthyroidismHyperthyroidism is a condition in which the thyroid gland produces excess thyroid hormone, accelerating the body's metabolism. Common findings include weight loss, heat intolerance, tremor, and rapid heartbeat; Graves disease is the most frequent cause.
HypoparathyroidismHypoparathyroidism is a condition in which the parathyroid glands produce too little parathyroid hormone (PTH), causing low blood calcium and high phosphate. It most often follows accidental damage to the parathyroid glands during thyroid surgery.
HypothyroidismHypothyroidism is a condition in which the thyroid gland produces too little thyroid hormone, slowing the body's metabolism. Common findings include fatigue, cold intolerance, weight gain, constipation, dry skin, and bradycardia.
ICD-10 diagnostic codingICD-10 is the 10th revision of the International Classification of Diseases, the standardized code set used to report diagnoses. In the United States, the ICD-10-CM version assigns alphanumeric codes to conditions for medical records and insurance claims.
Immunodeficiency disordersImmunodeficiency disorders are conditions in which part of the immune system is missing or dysfunctional, leaving the body vulnerable to recurrent, severe, or unusual infections. They are classified as primary (inherited) or secondary (acquired).
ImmunoglobulinsImmunoglobulins, or antibodies, are Y-shaped proteins produced by B cells and plasma cells that bind specific antigens to neutralize pathogens and mark them for destruction. The five classes are IgG, IgA, IgM, IgE, and IgD.
Increased intracranial pressureIncreased intracranial pressure is a rise in pressure inside the skull above the normal range of roughly 5–15 mmHg, caused by brain swelling, bleeding, tumors, or excess cerebrospinal fluid. It compresses brain tissue and reduces cerebral blood flow.
Infection controlInfection control is the set of practices healthcare workers use to prevent the spread of infectious organisms among patients, staff, and visitors. It includes hand hygiene, personal protective equipment, sterilization, and standard precautions.
InflammationInflammation is the body's protective response of vascularized tissue to injury or infection, marked by redness, heat, swelling, pain, and loss of function. It delivers immune cells and mediators to the damaged site and initiates healing.
Innate immunityInnate immunity is the body's built-in, first-line defense system that responds to pathogens within minutes to hours. It is nonspecific and has no memory, relying on barriers, phagocytes, natural killer cells, complement, and inflammation rather than antigen-specific recognition.
Intracellular vs. extracellular fluidIntracellular fluid (ICF) is the water inside cells — about two-thirds of total body water — while extracellular fluid (ECF) is the remaining third outside cells, divided between interstitial fluid and blood plasma.
Intravenous (IV) therapyIntravenous (IV) therapy is the administration of fluids, medications, blood products, or nutrition directly into a vein. It provides immediate absorption and precise control, making it the fastest route for delivering treatment.
Ischemic colitisIschemic colitis is inflammation and injury of the colon caused by inadequate blood flow. It typically affects older adults and classically presents with crampy abdominal pain followed by bloody diarrhea.
Ischemic heart disease (IHD)Ischemic heart disease (IHD) is a group of conditions caused by inadequate blood supply to the myocardium, most often from atherosclerotic narrowing of the coronary arteries. It encompasses angina pectoris, myocardial infarction, chronic ischemic heart disease, and sudden cardiac death.
Joint structure and functionJoint structure and function describes how the body's articulations are built and how they move. Joints are classified structurally as fibrous, cartilaginous, or synovial, and functionally by how much movement they allow.
Kiesselbach's plexusKiesselbach's plexus is a network of anastomosing arteries on the anterior nasal septum, located in Little's area. It is the most common source of anterior nosebleeds (epistaxis).
KinesiologyKinesiology is the study of human movement, combining anatomy, physiology, and biomechanics to explain how muscles, bones, and joints produce motion. It underpins fields including massage therapy, athletic training, physical therapy, and strength and conditioning.
Klebsiella pneumoniaeKlebsiella pneumoniae is an encapsulated, lactose-fermenting, gram-negative rod in the Enterobacteriaceae family. It is a normal gut colonizer that causes lobar pneumonia, urinary tract infections, and hospital-acquired infections, and it is a major source of multidrug resistance.
Korsakoff syndromeKorsakoff syndrome is a chronic memory disorder caused by severe thiamine (vitamin B1) deficiency, most often in the setting of long-term alcohol use. Its hallmarks are profound anterograde amnesia, retrograde amnesia, and confabulation.
Large intestine absorptionThe large intestine primarily absorbs water and electrolytes, especially sodium, from the indigestible material that leaves the small intestine. It also absorbs vitamins produced by gut bacteria, including vitamin K and some B vitamins, while compacting waste into feces.
Legionella pneumophilaLegionella pneumophila is a gram-negative bacterium that lives in warm water systems and causes Legionnaires' disease, a severe atypical pneumonia. It spreads by inhaled aerosols rather than person to person.
LeukemiaLeukemia is a malignancy of blood-forming cells in which abnormal white blood cell precursors proliferate in the bone marrow and spill into the bloodstream. Crowding of normal marrow produces anemia, infection risk, and bleeding.
Lingual lipaseLingual lipase is a fat-digesting enzyme secreted by glands of the tongue. It begins triglyceride breakdown in the mouth and, because it works best at acidic pH, continues acting in the stomach.
Listeria monocytogenesListeria monocytogenes is a gram-positive, facultative intracellular bacillus that causes foodborne listeriosis. It is notable for growing at refrigeration temperatures and for causing severe disease in pregnant patients, neonates, older adults, and immunocompromised people.
Liver spanLiver span is the vertical height of the liver measured at the right midclavicular line, typically by percussion. A normal adult liver span is roughly 6–12 cm; a span beyond that range suggests hepatomegaly or a shrunken liver.
Lung cancerLung cancer is a malignant tumor arising from the tissues of the lung, most often the bronchial epithelium. It is divided into small cell and non-small cell types, and it is the leading cause of cancer death worldwide.
Lymphatic systemThe lymphatic system is a network of vessels, nodes, and organs that returns excess interstitial fluid to the bloodstream, absorbs dietary fats from the intestine, and houses the immune cells that screen that fluid for pathogens.
LymphedemaLymphedema is chronic swelling caused by impaired lymphatic drainage, which allows protein-rich fluid to accumulate in the interstitial tissues. It most often affects an arm or leg and, over time, produces firm, non-pitting swelling with skin thickening.
LymphomaLymphoma is a cancer of lymphocytes that arises in lymphoid tissue such as lymph nodes, the spleen, and mucosa-associated lymphoid tissue. It is divided into Hodgkin lymphoma and the much larger group of non-Hodgkin lymphomas.
Lysosomal storage disordersLysosomal storage disorders are inherited diseases in which a missing or defective lysosomal enzyme allows its substrate to accumulate inside cells. The stored material progressively damages tissues, most often the brain, liver, spleen, and bone.
Macrocytic anemiasMacrocytic anemia is anemia in which red blood cells are abnormally large, with a mean corpuscular volume above roughly 100 fL. The major subdivision is megaloblastic, caused by impaired DNA synthesis, versus non-megaloblastic.
Malignancies of the thyroid glandMalignancies of the thyroid gland are cancers arising from the gland's follicular or parafollicular cells, most often presenting as a painless neck nodule. The four main types — papillary, follicular, medullary, and anaplastic — differ sharply in behavior and prognosis.
Mean arterial pressureMean arterial pressure (MAP) is the average pressure in a person's arteries over one cardiac cycle. Because it reflects the driving force for blood flow to the organs, it is the pressure value used to judge whether tissues are being adequately perfused.
Measures of associationMeasures of association are the statistics epidemiologists use to quantify the relationship between an exposure and an outcome, such as relative risk, odds ratio, and attributable risk. Which measure applies depends on the study design.
Measures of disease frequencyMeasures of disease frequency are the epidemiologic statistics that quantify how much disease exists in a population. The two core measures are incidence, which counts new cases arising over a period, and prevalence, which counts all existing cases at a point in time.
Mechanics of breathingThe mechanics of breathing describe how pressure changes created by respiratory muscles move air into and out of the lungs. Inspiration is active and driven mainly by the diaphragm, while quiet expiration is passive and driven by elastic recoil.
Mechanisms of action of antibioticsAntibiotics kill or inhibit bacteria by targeting structures and processes unique to bacterial cells — chiefly cell wall synthesis, protein synthesis, nucleic acid synthesis, folate metabolism, and cell membrane integrity.
MeiosisMeiosis is the cell division that produces gametes, taking one diploid cell through two rounds of division to yield four haploid daughter cells. It halves the chromosome number and shuffles genetic material, making it the main source of genetic variation between offspring.
Membrane-bound organellesMembrane-bound organelles are internal cell compartments enclosed by their own lipid bilayer, such as the nucleus, mitochondria, endoplasmic reticulum, Golgi apparatus, and lysosomes. Their presence is the defining feature that separates eukaryotic cells from prokaryotic ones.
MeningesThe meninges are the three connective tissue membranes — dura mater, arachnoid mater, and pia mater — that surround and protect the brain and spinal cord. Cerebrospinal fluid circulates between the arachnoid and pia in the subarachnoid space.
MenopauseMenopause is the permanent end of menstruation, diagnosed after 12 consecutive months without a period. It results from depletion of ovarian follicles, causing estrogen to fall while FSH and LH rise, typically around age 51.
Menstrual cycleThe menstrual cycle is the recurring hormonal sequence that matures an oocyte and prepares the uterine lining for implantation. It averages about 28 days and is described by parallel ovarian phases (follicular, ovulation, luteal) and uterine phases (menstrual, proliferative, secretory).
Mesenteric ischemiaMesenteric ischemia is inadequate blood flow to the intestines through the mesenteric vessels, causing abdominal pain and, if untreated, bowel infarction. It can be acute — a surgical emergency — or chronic, producing pain after meals.
Metabolic syndromeMetabolic syndrome is a cluster of conditions — abdominal obesity, high blood pressure, elevated fasting glucose, high triglycerides, and low HDL cholesterol — that together sharply raise the risk of type 2 diabetes and cardiovascular disease.
Mucor and RhizopusMucor and Rhizopus are environmental molds that cause mucormycosis, an aggressive opportunistic fungal infection. Both show broad, ribbon-like nonseptate hyphae with wide-angle branching and classically affect patients with diabetic ketoacidosis or immunosuppression.
Muscle actionsMuscle actions are the ways a muscle produces force: concentric (shortening), eccentric (lengthening under tension), and isometric (holding length constant). They describe how muscles create, control, and resist movement.
MycobacteriaMycobacteria are slow-growing, aerobic, acid-fast bacilli with a waxy mycolic acid cell wall. The genus includes Mycobacterium tuberculosis, M. leprae, and atypical species such as Mycobacterium avium complex.
Mycoplasma pneumoniaeMycoplasma pneumoniae is a tiny bacterium with no cell wall that causes atypical "walking pneumonia," most often in young people in close quarters. Its lack of a cell wall makes it invisible on Gram stain and resistant to penicillins.
MyocarditisMyocarditis is inflammation of the heart muscle (myocardium), most often caused by viral infection — classically coxsackievirus B. It can present with chest pain, arrhythmias, or heart failure, and may progress to dilated cardiomyopathy.
Naegleria fowleriNaegleria fowleri is a free-living amoeba found in warm freshwater that causes primary amebic meningoencephalitis (PAM), a rare but rapidly fatal brain infection acquired when contaminated water enters the nose.
NecrosisNecrosis is the death of cells or tissue caused by injury, such as loss of blood supply, infection, or toxins. Unlike apoptosis, it is an uncontrolled process that triggers inflammation in the surrounding tissue.
Necrotic tissueNecrotic tissue is tissue that has died as a result of injury, ischemia, or infection. Unlike programmed cell death, necrosis involves cell rupture and release of intracellular contents, which provokes an inflammatory response in the surrounding tissue.
Neisseria gonorrhoeaeNeisseria gonorrhoeae is a gram-negative diplococcus that causes gonorrhea, a sexually transmitted infection. It produces urethritis, cervicitis, pelvic inflammatory disease, septic arthritis, and neonatal conjunctivitis.
Neisseria meningitidisNeisseria meningitidis is a gram-negative diplococcus that colonizes the human nasopharynx and can cause bacterial meningitis and life-threatening meningococcemia, classically in adolescents and young adults living in close quarters.
NephrolithiasisNephrolithiasis is the formation of stones (calculi) in the kidneys. Stones form when minerals precipitate out of concentrated urine, and classically present with severe colicky flank pain radiating to the groin, often with blood in the urine.
Neurodegenerative disordersNeurodegenerative disorders are diseases marked by the progressive loss of neurons in the brain or spinal cord, producing gradually worsening cognitive or motor decline. Major examples include Alzheimer disease, Parkinson disease, Huntington disease, and ALS.
NeuropathiesNeuropathies are disorders of the peripheral nerves that disrupt sensory, motor, or autonomic signaling. Symptoms range from numbness, tingling, and burning pain to weakness, and diabetes is the most common cause.
NeurotransmitterA neurotransmitter is a chemical messenger that carries a signal from one neuron across a synapse to another neuron, muscle, or gland. Common examples include acetylcholine, dopamine, serotonin, norepinephrine, GABA, and glutamate.
NocardiaNocardia is a genus of gram-positive, branching, filamentous bacteria that are weakly (partially) acid-fast and strictly aerobic. Found in soil, it causes pulmonary, cutaneous, and disseminated nocardiosis, especially in immunocompromised patients.
Nucleus vs. nucleolusThe nucleus is the membrane-bound organelle that houses a eukaryotic cell's DNA, while the nucleolus is a dense, non-membrane-bound region inside the nucleus where ribosomal RNA is synthesized and ribosome subunits are assembled.
Obstructive lung diseaseObstructive lung disease is a group of respiratory conditions — including asthma, chronic bronchitis, emphysema, and bronchiectasis — in which airflow out of the lungs is limited. The hallmark is a reduced FEV1/FVC ratio on pulmonary function testing, reflecting difficulty exhaling.
OrthomyxovirusesOrthomyxoviruses are a family of enveloped viruses with segmented, negative-sense RNA genomes, best known for the influenza viruses. Their segmented genome allows reassortment, the mechanism behind antigenic shift and flu pandemics.
Oxygen saturationOxygen saturation is the percentage of hemoglobin in the blood that is carrying oxygen. Measured noninvasively by pulse oximetry (SpO2), a normal reading is roughly 95-100% in healthy individuals.
PancreasThe pancreas is a retroperitoneal gland behind the stomach with dual roles: its exocrine tissue secretes digestive enzymes into the duodenum, and its endocrine islets release insulin and glucagon to regulate blood glucose.
Papilloma and polyoma virusesPapillomaviruses and polyomaviruses are small, nonenveloped viruses with circular double-stranded DNA genomes that replicate in the host cell nucleus. Papillomaviruses cause warts and cervical cancer, while polyomaviruses such as JC and BK cause disease mainly in immunocompromised patients.
Paracentral lobuleThe paracentral lobule is the region on the medial surface of the cerebral hemisphere where the precentral and postcentral gyri continue over the midline. It contains the motor and sensory representation of the contralateral lower limb and perineum.
ParamyxoviridaeParamyxoviridae is a family of enveloped, negative-sense, single-stranded RNA viruses that includes measles, mumps, and the parainfluenza viruses. Its members spread by respiratory droplets and cause several classic childhood diseases.
Parkinson's diseaseParkinson's disease is a progressive neurodegenerative disorder caused by the loss of dopamine-producing neurons in the substantia nigra. It produces the cardinal motor signs of resting tremor, rigidity, bradykinesia, and postural instability, along with a wide range of non-motor symptoms.
ParturitionParturition is the physiological process of giving birth — the expulsion of the fetus and placenta from the uterus at the end of pregnancy, driven by coordinated hormonal and mechanical signals.
Pericarditis vs. endocarditisPericarditis is inflammation of the pericardium, the sac surrounding the heart, while endocarditis is inflammation — usually infection — of the heart's inner lining and valves. They differ in cause, presentation, and workup.
Peripheral nervous systemThe peripheral nervous system is everything in the nervous system outside the brain and spinal cord — the cranial and spinal nerves, their branches, and the associated ganglia. It carries sensory information inward and motor commands outward.
Pernicious anemiaPernicious anemia is a macrocytic anemia caused by vitamin B12 deficiency due to autoimmune destruction of gastric parietal cells, which eliminates the intrinsic factor needed to absorb B12 in the terminal ileum.
Pharyngeal archesThe pharyngeal arches are paired embryonic structures in the developing head and neck that give rise to specific muscles, skeletal elements, arteries, and cranial nerves. Five arches develop in humans, numbered 1, 2, 3, 4, and 6.
PhlebotomyPhlebotomy is the practice of puncturing a vein to collect blood, most often for laboratory testing. The term also covers therapeutic blood removal used to treat conditions of iron or red cell excess.
Physiologic hypertrophyPhysiologic hypertrophy is an increase in cell size, and therefore organ size, that occurs as a normal adaptive response to increased workload or hormonal stimulation. Skeletal muscle enlargement from resistance training and uterine enlargement during pregnancy are the classic examples.
PicornavirusPicornaviruses are a family of small, nonenveloped viruses with a single-stranded, positive-sense RNA genome. The family includes poliovirus, echovirus, rhinovirus, coxsackievirus, and hepatitis A virus.
Pituitary hormonesThe pituitary hormones are the chemical messengers released by the pituitary gland: growth hormone, prolactin, TSH, ACTH, FSH, and LH from the anterior lobe, plus antidiuretic hormone and oxytocin released from the posterior lobe.
PlacentaThe placenta is a temporary organ that forms during pregnancy to connect the fetus to the uterine wall. It handles gas exchange, nutrient delivery, waste removal, and hormone production, and it is expelled after birth as the afterbirth.
PlasmodiumPlasmodium is the genus of protozoan parasites that causes malaria. It is transmitted by the bite of a female Anopheles mosquito and infects liver cells and then red blood cells, producing cyclical fever, hemolytic anemia, and splenomegaly.
Platelet disordersPlatelet disorders are conditions in which platelets are too few (thrombocytopenia) or function abnormally, impairing primary hemostasis. They typically cause mucocutaneous bleeding such as petechiae, purpura, epistaxis, and easy bruising.
Pleural effusionA pleural effusion is an abnormal accumulation of fluid in the pleural space between the lung and the chest wall. The excess fluid compresses lung tissue and causes shortness of breath, and it is classified as either a transudate or an exudate.
Polycythemia veraPolycythemia vera is a myeloproliferative neoplasm in which a clonal bone marrow stem cell overproduces red blood cells, and often platelets and white cells as well. Nearly all cases carry a JAK2 mutation, and the thickened blood raises the risk of thrombosis.
PonsThe pons is the middle portion of the brainstem, located between the midbrain and the medulla oblongata. It relays signals between the cerebrum and the cerebellum and helps regulate breathing, sleep, and arousal.
PorphyriasThe porphyrias are a group of disorders caused by enzyme defects in the heme synthesis pathway. Intermediates accumulate upstream of the blocked step, producing either neurovisceral attacks or blistering photosensitivity depending on which enzyme is affected.
Post-streptococcal glomerulonephritis (PSGN)Post-streptococcal glomerulonephritis (PSGN) is an immune complex-mediated kidney disease that follows infection with group A Streptococcus, classically presenting with dark urine, edema, and hypertension. It most often affects children weeks after strep throat or a skin infection.
Pressure-volume loopA pressure-volume loop is a graph of left ventricular pressure against left ventricular volume across one cardiac cycle. Its width represents stroke volume, and its shape shifts predictably with changes in preload, afterload, and contractility.
Primary somatosensory cortexThe primary somatosensory cortex is the brain region in the postcentral gyrus of the parietal lobe that processes touch, pressure, pain, temperature, vibration, and proprioception from the opposite side of the body.
Procedural coding (CPT)Procedural coding is the process of assigning standardized numeric codes to the medical services and procedures a provider performs. In the United States it uses the Current Procedural Terminology (CPT) code set, maintained by the American Medical Association.
ProprioceptorsProprioceptors are sensory receptors located in muscles, tendons, and joints that detect body position, movement, and muscular tension. They supply the nervous system with the continuous feedback needed to control posture, coordinate movement, and protect tissue from excessive force.
ProstateThe prostate is a walnut-sized male accessory sex gland that sits below the bladder and surrounds the prostatic urethra. It secretes an alkaline fluid that makes up a substantial fraction of semen volume and supports sperm motility.
Protein structureProtein structure describes the four levels of organization in a protein: the amino acid sequence, local folding patterns, the overall three-dimensional shape, and the assembly of multiple chains. Each level builds on the one below it and determines what the protein can do.
PseudomonasPseudomonas is a genus of gram-negative, aerobic, oxidase-positive bacilli. Its key species, Pseudomonas aeruginosa, is an opportunistic pathogen known for hospital-acquired infections, blue-green pigment, and notable antibiotic resistance.
Pulmonary capillary wedge pressure (PCWP)Pulmonary capillary wedge pressure (PCWP) is the pressure measured by wedging a balloon-tipped catheter in a pulmonary artery branch. Normal PCWP is roughly 4–12 mmHg, and it serves as an indirect estimate of left atrial pressure.
Refractive errorsRefractive errors are vision problems in which the eye fails to focus light precisely on the retina, producing blurred vision. The main types are myopia, hyperopia, astigmatism, and presbyopia, all correctable with lenses.
Regulation of breathingBreathing is regulated by respiratory centers in the medulla and pons that set the rhythm of ventilation, adjusted by chemoreceptors that sense carbon dioxide, pH, and oxygen levels in the blood and cerebrospinal fluid.
Relative risk reduction (RRR)Relative risk reduction (RRR) is the proportional decrease in an outcome's risk in a treated group compared with a control group. It is calculated as (control event rate − experimental event rate) / control event rate, or equivalently 1 − relative risk.
Renal failureRenal failure is the loss of the kidneys' ability to filter waste and regulate fluid and electrolytes. It occurs as acute kidney injury, which develops rapidly and may be reversible, or chronic kidney disease, a progressive and irreversible decline.
Renal tubulesThe renal tubules are the segments of the nephron — proximal tubule, loop of Henle, distal tubule, and collecting duct — that convert glomerular filtrate into urine by reabsorbing needed substances and secreting wastes.
Respiratory failure and ARDSAcute respiratory distress syndrome (ARDS) is a form of acute hypoxemic respiratory failure in which diffuse injury to the alveolar-capillary membrane causes protein-rich, noncardiogenic pulmonary edema and severe impairment of oxygenation.
Restrictive lung diseaseRestrictive lung disease is a group of conditions in which the lungs cannot fully expand, reducing total lung capacity. Causes range from pulmonary fibrosis to chest wall and neuromuscular problems, and spirometry shows a normal or elevated FEV1/FVC ratio.
RickettsiaRickettsia is a genus of obligate intracellular, gram-negative bacteria transmitted by arthropod vectors such as ticks, lice, and mites. Rickettsial diseases include Rocky Mountain spotted fever and epidemic typhus, and are treated with doxycycline.
SalmonellaSalmonella is a genus of gram-negative bacilli that causes foodborne gastroenteritis and, in the case of Salmonella typhi, typhoid fever. It is a leading cause of bacterial food poisoning worldwide.
Seizure disordersSeizure disorders are neurological conditions marked by seizures — sudden episodes of abnormal, excessive electrical activity in the brain. Epilepsy is diagnosed when seizures are recurrent and unprovoked.
Sensitivity and specificitySensitivity is the proportion of people with a disease that a test correctly identifies as positive, while specificity is the proportion of people without the disease that it correctly identifies as negative. Together they describe a diagnostic test's accuracy.
Serotonin syndromeSerotonin syndrome is a potentially life-threatening drug reaction caused by excess serotonergic activity, usually from combining serotonergic medications. It presents with the triad of altered mental status, autonomic instability, and neuromuscular abnormalities such as clonus and hyperreflexia.
Sickle cell diseaseSickle cell disease is an inherited blood disorder in which a mutation in the beta-globin gene produces abnormal hemoglobin S, causing red blood cells to deform into rigid sickle shapes that block small vessels and break down prematurely.
Sideroblastic anemiaSideroblastic anemia is an anemia caused by defective heme synthesis, in which iron accumulates in the mitochondria of red cell precursors and forms ring sideroblasts visible on bone marrow staining. It can be inherited or acquired from causes such as alcohol, lead, or vitamin B6 deficiency.
SilicosisSilicosis is an occupational lung disease caused by inhaling crystalline silica dust, seen in workers such as miners, sandblasters, and stone cutters. It produces fibrotic nodules, classically in the upper lung lobes, and increases the risk of tuberculosis.
Skeletal muscle contractionSkeletal muscle contraction is the process by which muscle fibers generate force, explained by the sliding filament theory: myosin heads pull actin filaments toward the center of the sarcomere, shortening the muscle.
Sleep disordersSleep disorders are conditions that disrupt the quality, timing, or amount of sleep, including insomnia, obstructive sleep apnea, narcolepsy, parasomnias, and circadian rhythm disorders.
Spinal cord disordersSpinal cord disorders are conditions that damage the ascending sensory or descending motor tracts of the spinal cord, producing characteristic patterns of weakness and sensory loss. Classic examples include syringomyelia, ALS, and subacute combined degeneration.
Spinal cord tracts and lesionsSpinal cord tracts are bundles of nerve fibers that carry sensory information up to the brain (ascending tracts) or motor commands down from the brain (descending tracts). Damage to specific tracts produces predictable patterns of sensory or motor loss.
Standard and transmission-based precautionsStandard precautions are infection control practices applied to every patient, while transmission-based precautions add contact, droplet, or airborne measures for patients known or suspected to carry specific infectious organisms.
StaphylococciStaphylococci are gram-positive cocci that grow in grape-like clusters and are catalase-positive. The genus includes Staphylococcus aureus, a major cause of skin, bone, and bloodstream infections, along with several coagulase-negative species.
Sterile techniqueSterile technique, also called surgical asepsis, is the set of practices that keeps objects and areas completely free of all microorganisms during procedures. It is required for surgery, injections, catheter insertion, and sterile dressing changes.
StreptococciStreptococci are gram-positive cocci that grow in chains and are catalase-negative. The genus includes major human pathogens such as Streptococcus pyogenes, Streptococcus agalactiae, and Streptococcus pneumoniae.
StrongyloidesStrongyloides stercoralis is a parasitic roundworm (nematode) that infects humans when larvae in contaminated soil penetrate the skin. It is unique for its autoinfection cycle, which can persist for decades and cause life-threatening hyperinfection in immunocompromised patients.
Systemic mycosesSystemic mycoses are deep fungal infections caused by dimorphic fungi that are acquired by inhaling spores from the environment. Each organism is tied to a specific geographic region, and infection usually begins in the lungs before it can disseminate.
T and B lymphocytesT and B lymphocytes are the white blood cells that carry out adaptive immunity. T cells mature in the thymus and drive cell-mediated responses, while B cells mature in the bone marrow and produce antibodies.
T cell activationT cell activation is the process by which a naive T lymphocyte recognizes antigen and becomes a functional effector cell. It requires two signals: T cell receptor binding to antigen presented on MHC, plus a costimulatory signal.
TaeniaTaenia is a genus of parasitic tapeworm that infects humans through undercooked meat. Taenia solium comes from pork and Taenia saginata from beef, and only T. solium can also cause cysticercosis.
ThalamusThe thalamus is a paired mass of gray matter in the diencephalon that acts as the relay station between incoming sensory pathways and the cerebral cortex. Every major sensory system except olfaction synapses there before reaching conscious awareness.
ThalassemiaThalassemia is an inherited anemia caused by reduced or absent production of the alpha or beta globin chains of hemoglobin. The imbalance in chain synthesis produces small, pale red blood cells and, in severe forms, chronic hemolysis requiring lifelong transfusion.
Therapeutic communicationTherapeutic communication is a set of purposeful, patient-centered techniques clinicians use to build trust and encourage patients to express their thoughts and feelings. It emphasizes listening and exploring rather than advising or reassuring.
Thrombophilias (hypercoagulable disorders)Thrombophilia is an increased tendency to form blood clots, caused by inherited or acquired abnormalities in the coagulation system. Affected patients develop venous thromboembolism — most often deep vein thrombosis and pulmonary embolism — at unusually young ages or without a clear provoking cause.
Thyroid and parathyroid glandsThe thyroid is a butterfly-shaped gland in the anterior neck that produces hormones regulating metabolism, while the four parathyroid glands on its posterior surface secrete parathyroid hormone to control calcium. They sit together anatomically but serve distinct endocrine functions.
Thyroid hormonesThyroid hormones are thyroxine (T4) and triiodothyronine (T3), iodine-containing hormones made by the thyroid gland that set the body's basal metabolic rate. They also support growth, brain development, and the cardiovascular response to catecholamines.
Toxoplasma gondiiToxoplasma gondii is an intracellular protozoan parasite that causes toxoplasmosis. Cats are the definitive host, and humans are infected by ingesting oocysts from cat feces or tissue cysts in undercooked meat, or through transmission across the placenta.
Transcription and translationTranscription is the process of copying a gene's DNA sequence into messenger RNA, and translation is the process of reading that mRNA to build a protein. Together they carry out the central dogma of molecular biology: DNA to RNA to protein.
Transplant rejectionTransplant rejection is the recipient's immune response against a transplanted organ or tissue, triggered by donor antigens the immune system recognizes as foreign. It is classified as hyperacute, acute, or chronic based on timing and mechanism.
Treponema pallidumTreponema pallidum is the spiral-shaped bacterium that causes syphilis. It is too thin to be seen on Gram stain, cannot be grown on standard laboratory media, and produces a disease that progresses through primary, secondary, latent, and tertiary stages if untreated.
TrichomonasTrichomonas vaginalis is a flagellated protozoan parasite that causes trichomoniasis, a common sexually transmitted infection. It exists only as a motile trophozoite, with no cyst stage, and is treated with metronidazole.
TrichurisTrichuris trichiura, known as whipworm, is an intestinal roundworm that infects the large intestine after ingestion of eggs from contaminated soil. Heavy infection causes bloody diarrhea, anemia, and rectal prolapse in children.
Trinucleotide repeat disordersTrinucleotide repeat disorders are genetic diseases caused by an abnormally expanded three-nucleotide sequence within a gene. The repeat count grows across generations, so the disease often appears earlier and more severely in each successive generation.
Tubular reabsorptionTubular reabsorption is the process by which the kidney's nephrons return filtered water and solutes from the tubular fluid back into the blood. It reclaims most of what the glomerulus filters, including nearly all glucose, amino acids, and sodium.
Tubulointerstitial diseasesTubulointerstitial diseases are kidney disorders that primarily damage the renal tubules and the interstitial tissue around them, rather than the glomeruli. Major examples include acute tubular necrosis, acute interstitial nephritis, and pyelonephritis.
Types of emboliAn embolism occurs when material travels through the bloodstream and lodges in a vessel, blocking blood flow. Most emboli are dislodged blood clots, but fat, air, amniotic fluid, tumor fragments, and infected material can all embolize.
Types of study designTypes of study design are the frameworks researchers use to investigate health questions, divided into observational designs — case reports, cross-sectional, case-control, and cohort studies — and experimental designs like randomized controlled trials.
Umbilical veinThe umbilical vein is the vessel that carries oxygenated, nutrient-rich blood from the placenta to the fetus. It is the exception to the usual rule that veins carry deoxygenated blood.
Urea cycle disordersUrea cycle disorders are inherited enzyme deficiencies that block the conversion of ammonia into urea, causing ammonia to accumulate in the blood. Hyperammonemia is the common feature, and it is neurotoxic.
UrinalysisUrinalysis is a laboratory examination of urine using physical, chemical, and microscopic methods. It screens for urinary tract infections, kidney disease, diabetes, and other conditions from a simple urine specimen.
Urinary tract infection (UTI)A urinary tract infection (UTI) is a bacterial infection of the urinary system, most often the bladder. Escherichia coli causes the large majority of cases, and typical symptoms include painful urination, urgency, and frequency.
Vaginal lymphatic drainageVaginal lymphatic drainage follows a segmental pattern: the upper vagina drains to the external and internal iliac nodes, while the lower vagina, below the hymen, drains to the superficial inguinal nodes.
Valvular heart diseaseValvular heart disease is any condition in which one of the heart's four valves is damaged or dysfunctional, either failing to open fully (stenosis) or failing to close completely (regurgitation), disrupting normal blood flow.
VasculitisVasculitis is inflammation of blood vessel walls, which narrows or damages the vessels and starves downstream tissues of blood. The vasculitides are classified by the size of the vessels they attack — large, medium, or small.
VenipunctureVenipuncture is the procedure of puncturing a vein with a needle to collect a blood specimen or gain venous access. It is the most common method of blood collection, typically performed on veins in the antecubital area of the arm.
Vibrio choleraeVibrio cholerae is the comma-shaped, gram-negative bacterium that causes cholera. Its toxin drives massive chloride and water secretion in the small intestine, producing profuse rice-water diarrhea that can be fatal without rehydration.
Virulence factorsVirulence factors are the molecules and structures a pathogen uses to colonize a host, evade the immune system, and cause disease — including capsules, pili, toxins, and tissue-degrading enzymes.
Virus structure and life cycleThe virus life cycle is the sequence a virus follows to reproduce inside a host cell: attachment, entry, uncoating, replication and protein synthesis, assembly, and release. Viruses are obligate intracellular parasites built from a genome and a protein capsid.