1500+ medical terms, defined.

Acid phosphatase is an enzyme optimally active at acidic pH, historically used as a serum marker for prostate cancer before being replaced by PSA. It is also notably elevated in Gaucher disease and can be used forensically to detect semi...

The mesentery is a double layer of peritoneum that suspends the small intestine and other abdominal organs from the posterior abdominal wall, providing a conduit for blood vessels, lymphatics, and nerves. It allows mobility of the bowel...

Rotor syndrome is a rare, benign autosomal recessive disorder causing conjugated (and unconjugated) hyperbilirubinemia due to impaired hepatic storage and reflux of bilirubin into plasma. Unlike Dubin-Johnson syndrome, the liver is not p...

Somatic hypermutation is a process in activated B cells that introduces point mutations into the variable regions of immunoglobulin genes, enabling affinity maturation of antibodies. It occurs in the germinal centers of lymph nodes and i...

Affinity maturation is the process by which B cells produce antibodies with progressively higher affinity for antigen over the course of an immune response. It occurs in germinal centers of lymph nodes and is driven by somatic hypermutat...

The JAK-STAT pathway is a signal transduction cascade used by cytokine receptors (lacking intrinsic kinase activity) to transmit signals from the cell surface to the nucleus, regulating gene transcription. Janus kinases (JAKs) phosphoryl...

Argininosuccinate synthetase (ASS1) catalyzes the third step of the urea cycle, combining citrulline and aspartate to form argininosuccinate. Deficiency causes citrullinemia type I, an autosomal recessive urea cycle disorder leading to h...

Chronic mesenteric ischemia ('intestinal angina') results from atherosclerotic narrowing of ≥2 of the three mesenteric arteries (celiac, SMA, IMA), causing postprandial abdominal pain due to inadequate blood flow to meet increased metabo...

Argyll Robertson pupil is a classic sign of neurosyphilis (tertiary syphilis) characterized by bilateral small, irregular pupils that constrict normally with accommodation (near stimulus) but fail to constrict to light—'accommodates but...

Hypertensive retinopathy refers to retinal vascular damage caused by chronic or acute severe hypertension. Findings on fundoscopic exam include arteriolar narrowing, AV nicking, cotton-wool spots, flame-shaped hemorrhages, and in severe/...

Endocytosis is a cellular process in which the plasma membrane invaginates to engulf extracellular material, forming an intracellular vesicle. It includes phagocytosis (cell eating, for large particles/pathogens), pinocytosis (cell drink...

West Nile virus (WNV) is an arbovirus of the Flaviviridae family, transmitted to humans via Culex mosquito bites, with birds serving as the natural reservoir. Most infections are asymptomatic, but it can cause febrile illness, meningitis...

Mosaicism is the presence of two or more genetically distinct cell populations derived from a single fertilized zygote, typically arising from a mitotic error after fertilization. It differs from chimerism, which involves cells from two...

The portal triad refers to the three structures that run together within the portal tract (Glisson's capsule) at the corners of hepatic lobules: a branch of the portal vein, a branch of the hepatic artery, and a bile duct(ule). It is a k...

Propionyl-CoA carboxylase (PCC) is a biotin-dependent mitochondrial enzyme that converts propionyl-CoA to D-methylmalonyl-CoA, a key step in the catabolism of odd-chain fatty acids, cholesterol, and certain amino acids (valine, isoleucin...

A branchial cleft cyst is a congenital neck mass arising from incomplete obliteration of the branchial (pharyngeal) apparatus during embryonic development, most commonly the second branchial cleft. It classically presents as a smooth, no...

Primary syphilis is the first stage of infection with Treponema pallidum, characterized by a single, painless, indurated ulcer (chancre) at the site of inoculation, typically appearing 3 weeks after exposure. It is accompanied by nontend...

A thyroglossal duct cyst is a congenital midline neck mass resulting from persistence of the thyroglossal duct, the embryologic tract along which the thyroid descends from the foramen cecum at the tongue base to its final pretracheal pos...

Dihydrotestosterone (DHT) is a potent androgen formed from testosterone via 5α-reductase. It is essential for male external genitalia differentiation in utero and drives prostate growth and male pattern hair changes postnatally. DHT is m...

The Fc (fragment crystallizable) region is the stem/tail portion of an antibody (immunoglobulin) molecule, composed of the constant domains of two heavy chains. It mediates effector functions of antibodies by binding to Fc receptors on i...

Left-sided heart failure occurs when the left ventricle fails to pump blood effectively, leading to pulmonary congestion (due to backup into pulmonary circulation) and/or reduced systemic perfusion. It presents classically with dyspnea,...

A barium swallow (esophagram) is a fluoroscopic contrast imaging study used to evaluate the oropharynx and esophagus for structural and functional abnormalities. The patient swallows barium sulfate contrast while radiographic images are...

Zika virus is a flavivirus transmitted primarily by Aedes mosquitoes, also spread via sexual contact and vertically from mother to fetus. Infection is often asymptomatic or causes mild febrile illness with rash, conjunctivitis, and arthr...

Autism spectrum disorder (ASD) is a neurodevelopmental disorder characterized by persistent deficits in social communication/interaction and restricted, repetitive patterns of behavior, interests, or activities, with onset in early child...

Direct (conjugated) bilirubin is water-soluble bilirubin that has been conjugated with glucuronic acid by hepatocyte UDP-glucuronosyltransferase (UGT1A1). Elevated direct bilirubin indicates impaired excretion into bile (intrahepatic cho...

Coccidioidomycosis ('Valley Fever') is a fungal infection caused by Coccidioides immitis/posadasii, endemic to the southwestern US (San Joaquin Valley, Arizona) and Central/South America. It is acquired by inhaling arthroconidia from dis...

Histoplasmosis is a fungal infection caused by Histoplasma capsulatum, a dimorphic fungus found in soil contaminated with bird or bat droppings, endemic to the Ohio and Mississippi River valleys. Infection occurs via inhalation of microc...

Blastomyces dermatitidis is a thermally dimorphic fungus endemic to the Mississippi and Ohio River valleys and Great Lakes region, causing blastomycosis. It exists as a mold in soil (25°C) and yeast in tissue (37°C), and infection typica...

Blastomycosis is a systemic fungal infection caused by Blastomyces dermatitidis, a dimorphic fungus endemic to the Midwest/South-Central and Great Lakes regions of North America (overlapping with Histoplasma). Infection occurs via inhala...

The renal medulla is the inner region of the kidney, consisting of pyramids that contain the loops of Henle, vasa recta, and collecting ducts. It is critical for establishing the medullary osmotic gradient, which enables urine concentrat...

Immune thrombocytopenic purpura (ITP) is an autoimmune disorder characterized by isolated thrombocytopenia due to antiplatelet antibodies (typically against GPIIb/IIIa) causing platelet destruction in the spleen. It presents as mucocutan...

Primase is an RNA polymerase enzyme that synthesizes short RNA primers to initiate DNA replication, since DNA polymerases cannot start synthesis de novo. It works with helicase as part of the primosome complex and is essential for both l...

Topoisomerases are enzymes that resolve DNA supercoiling and topological problems during replication and transcription by transiently cutting and rejoining DNA strands. Bacterial topoisomerase II (DNA gyrase) is targeted by fluoroquinolo...

Prasugrel is a third-generation thienopyridine that irreversibly inhibits the P2Y12 ADP receptor on platelets, preventing platelet aggregation. It is used with aspirin (dual antiplatelet therapy) in acute coronary syndrome patients under...

Empiric therapy is the initiation of treatment—most commonly antibiotics—based on clinical presentation and likely pathogens before definitive microbiologic identification or lab confirmation is available. It is guided by the most probab...

Rouleaux formation refers to red blood cells stacking together like coins on peripheral blood smear, caused by increased plasma proteins (especially fibrinogen and immunoglobulins) that decrease the negative surface charge repulsion betw...

Dressler syndrome is an autoimmune-mediated pericarditis that occurs weeks to months (typically 2-10 weeks) after myocardial infarction, cardiac surgery, or cardiac trauma. It presents with fever, pleuritic chest pain, and a friction rub...

Eculizumab is a humanized monoclonal antibody that inhibits terminal complement protein C5, preventing its cleavage into C5a and C5b, thereby blocking formation of the membrane attack complex (MAC, C5b-9). It is primarily used to treat p...

Takotsubo cardiomyopathy (stress-induced or 'broken heart' cardiomyopathy) is a transient left ventricular dysfunction typically triggered by acute emotional or physical stress. It classically presents with chest pain, ECG changes mimick...

Stress (Takotsubo) cardiomyopathy is a transient, reversible left ventricular systolic dysfunction triggered by acute emotional or physical stress, mimicking acute myocardial infarction but occurring in the absence of significant coronar...

The caudate nucleus is a C-shaped subcortical structure that is part of the basal ganglia and dorsal striatum, primarily involved in motor control, procedural learning, and cognitive/executive functions. It works closely with the putamen...

Aminotransferases (transaminases) are enzymes that catalyze the transfer of an amino group between an amino acid and an α-keto acid, playing a key role in amino acid metabolism and gluconeogenesis. ALT (alanine aminotransferase) and AST...

Mallory bodies (Mallory-Denk bodies) are intracytoplasmic eosinophilic inclusions found in hepatocytes, composed of damaged keratin intermediate filaments aggregated with ubiquitin. They are classically associated with alcoholic liver di...

Calcium channel blockers (CCBs) are antihypertensive/antianginal drugs that block L-type voltage-gated calcium channels in vascular smooth muscle and cardiac tissue, causing vasodilation and reduced cardiac contractility/conduction. They...

Methylprednisolone is an intermediate-acting synthetic glucocorticoid used for its potent anti-inflammatory and immunosuppressive effects. It is commonly used in acute exacerbations of autoimmune and inflammatory conditions (e.g., MS fla...

Angiotensin-converting enzyme (ACE) is a peptidase found primarily on pulmonary vascular endothelium that converts angiotensin I to angiotensin II and degrades bradykinin. It is a key regulator of blood pressure, fluid balance, and vascu...

A Sister Mary Joseph nodule is a firm, palpable periumbilical nodule representing metastasis from an intra-abdominal or pelvic malignancy, most commonly gastric cancer. It signifies advanced-stage, often incurable disease. Recognition is...

Simple columnar epithelium consists of a single layer of tall, column-shaped cells with basally located, elongated nuclei. It is specialized for absorption and secretion, and is found lining the GI tract from stomach to anus, the gallbla...

Gastric inhibitory peptide (GIP), also known as glucose-dependent insulinotropic polypeptide, is an incretin hormone secreted by K cells in the duodenum and jejunum. It stimulates insulin release in response to oral glucose/fat intake an...

Trousseau sign is the carpal spasm (main d'accoucheur - 'obstetrician's hand' posture) induced by inflating a blood pressure cuff above systolic pressure for 3 minutes, indicating latent tetany due to hypocalcemia. It is a classic bedsid...