1500+ medical terms, defined.

A chromosomal translocation is a structural chromosomal abnormality in which a segment of one chromosome breaks off and attaches to another chromosome. Translocations can be balanced (no net loss of genetic material, usually phenotypical...

Appendectomy is the surgical removal of the vermiform appendix, most commonly performed to treat acute appendicitis. It can be done via open or laparoscopic approach, with laparoscopic being preferred in uncomplicated cases due to faster...

Pili (fimbriae) are hair-like proteinaceous appendages on the surface of many bacteria that mediate adhesion to host cells and surfaces, and in some cases enable DNA transfer (sex/conjugative pili). They are key virulence factors distinc...

The ligamentum arteriosum is a fibrous remnant of the fetal ductus arteriosus, connecting the pulmonary trunk to the aortic arch (just distal to the left subclavian artery). It normally closes functionally within hours to days after birt...

The platelet plug is the initial hemostatic response to vascular injury, formed via platelet adhesion, activation, and aggregation at the site of endothelial damage. It constitutes primary hemostasis, occurring within seconds to minutes,...

Meiosis is a specialized type of cell division that produces four haploid gametes (sperm or oocytes) from a single diploid germ cell, reducing chromosome number by half. It involves two rounds of division (Meiosis I and II) preceded by o...

The cricothyroid muscle is the only tensor muscle of the larynx, responsible for elongating and tensing the vocal cords to increase pitch. It is innervated by the external branch of the superior laryngeal nerve, distinguishing it from al...

Methylmalonyl-CoA mutase is a vitamin B12 (cobalamin)-dependent mitochondrial enzyme that converts methylmalonyl-CoA to succinyl-CoA, linking odd-chain fatty acid and amino acid catabolism to the TCA cycle. Deficiency of this enzyme or i...

Vasoactive intestinal peptide (VIP) is a neuropeptide secreted by neurons in the GI tract and pancreatic islet delta cells (or non-beta islet cells) that stimulates intestinal water and electrolyte secretion, relaxes GI smooth muscle and...

LCAT (Lecithin-Cholesterol Acyltransferase) is a plasma enzyme that esterifies free cholesterol on nascent HDL particles, allowing HDL to mature and enabling reverse cholesterol transport. LCAT deficiency causes accumulation of free chol...

Gram-negative rods (GNRs) are bacilli-shaped bacteria that stain pink/red on Gram stain due to a thin peptidoglycan layer and outer membrane containing lipopolysaccharide (LPS/endotoxin). They are classified by lactose fermentation, oxid...

Isoleucine is a branched-chain amino acid (BCAA) that is nutritionally essential (must be obtained from diet) and glucogenic/ketogenic in its metabolism. It is metabolized alongside leucine and valine by branched-chain alpha-ketoacid deh...

Leucine is a branched-chain amino acid (BCAA) that is nutritionally essential and purely ketogenic (does not contribute to gluconeogenesis). It plays a key role in muscle protein synthesis via mTOR pathway activation and is degraded via...

Calcium oxalate stones are the most common type of kidney stone (~80% of cases), forming due to hypercalciuria, hyperoxaluria, or hypocitraturia. They appear as radiopaque, envelope-shaped or dumbbell-shaped crystals on urinalysis and im...

Restrictive cardiomyopathy (RCM) is characterized by stiff, non-compliant ventricular walls that impair diastolic filling while systolic function remains relatively preserved. It leads to elevated filling pressures and biventricular dias...

Behçet disease is a systemic vasculitis of unknown etiology characterized by the classic triad of recurrent oral aphthous ulcers, genital ulcers, and uveitis. It is associated with HLA-B51 and is most common in populations along the anci...

Bile acids are amphipathic steroid molecules synthesized from cholesterol in the liver, essential for emulsification and absorption of dietary fats and fat-soluble vitamins (A, D, E, K). Primary bile acids (cholic acid, chenodeoxycholic...

Histidine is an essential amino acid that serves as the precursor for histamine synthesis via histidine decarboxylase. It is also a key component of hemoglobin's oxygen-binding site and buffers blood pH due to its imidazole side chain.

Bronchiolitis is an acute viral lower respiratory tract infection causing inflammation and obstruction of the bronchioles, most commonly affecting infants and children under 2 years old. Respiratory syncytial virus (RSV) is the leading c...

Glucagonoma is a rare, glucagon-secreting neuroendocrine tumor arising from pancreatic alpha cells, most commonly located in the pancreatic tail. It classically presents with the '4 D's': Dermatitis (necrolytic migratory erythema), Diabe...

The pleura is a thin, double-layered serous membrane that lines the lungs (visceral pleura) and thoracic cavity (parietal pleura), enclosing a fluid-filled potential space (pleural cavity) that facilitates lung expansion during respirati...

Urothelium (transitional epithelium) is a specialized stratified epithelium lining most of the urinary tract, from the renal pelvis to the proximal urethra. It is uniquely adapted to withstand stretch and hypertonic, urea-rich urine, pro...

Cytochrome P450 (CYP450) enzymes are a superfamily of heme-containing enzymes located primarily in the liver (smooth ER) that catalyze phase I oxidative metabolism of drugs, toxins, and endogenous compounds (steroids, bile acids). They a...

The brachial plexus is a network of nerves formed by the ventral rami of C5-T1 that innervates the upper limb, providing motor and sensory function to the shoulder, arm, forearm, and hand. It is organized into Roots, Trunks, Divisions, C...

Focal Segmental Glomerulosclerosis (FSGS) is a leading cause of nephrotic syndrome in adults and the most common cause in African Americans, characterized by segmental scarring of some (focal) glomeruli in some (segmental) areas. It pres...

Kupffer cells are specialized macrophages residing in the liver, primarily located within the hepatic sinusoids. They are part of the reticuloendothelial (mononuclear phagocyte) system and play a key role in clearing pathogens, damaged c...

A thoracic aortic aneurysm (TAA) is a permanent, localized dilation of the thoracic aorta (>50% increase in diameter compared to normal, typically >4.5 cm). Common causes include cystic medial degeneration, hypertension, atherosclerosis,...

Pulsus parvus et tardus refers to a weak (parvus) and delayed (tardus) arterial pulse, classically seen in severe aortic stenosis. It reflects reduced stroke volume and prolonged left ventricular ejection time due to obstruction of blood...

A friction rub is a scratchy, grating, or leathery sound heard on auscultation caused by inflamed serosal surfaces (pericardium or pleura) rubbing against each other. Pericardial friction rubs are heard best at the left sternal border wi...

Hypoxic pulmonary vasoconstriction (HPV) is a unique physiologic response in which alveolar hypoxia causes local pulmonary arteriolar constriction, diverting blood flow away from poorly ventilated lung regions toward better-ventilated ar...

The proton gradient (proton motive force) is an electrochemical gradient of H+ ions across the inner mitochondrial membrane, generated by the electron transport chain (ETC). It drives ATP synthesis via ATP synthase (oxidative phosphoryla...

ATP synthase is a mitochondrial inner membrane enzyme complex that synthesizes ATP from ADP and inorganic phosphate, using the proton-motive force generated by the electron transport chain. It represents Complex V of oxidative phosphoryl...

The adventitia (also called tunica externa) is the outermost connective tissue layer surrounding blood vessels, and by extension, other organs. It consists primarily of collagen and elastic fibers and provides structural support, anchori...

Distal renal tubular acidosis (Type 1 RTA) is caused by an inability of the α-intercalated cells in the collecting duct to secrete H+ into the urine, leading to impaired urinary acidification. This results in a non-anion gap metabolic ac...

A transudate is a low-protein, low-cellularity fluid that accumulates in body cavities or interstitium due to imbalances in hydrostatic and oncotic pressure, rather than inflammation. It is typically clear, has low specific gravity (<1.0...

Congo red is a histological stain used to detect amyloid deposits in tissue. Under polarized light, amyloid-bound Congo red exhibits characteristic apple-green birefringence. This is the gold-standard confirmatory test for amyloidosis.

Cholestyramine is a bile acid-binding resin used to lower LDL cholesterol and treat pruritus associated with cholestasis. It works by binding bile acids in the intestine, preventing their reabsorption and increasing their fecal excretion...

Secondary (reactivation) tuberculosis occurs when latent Mycobacterium tuberculosis reactivates, typically due to waning immunity (e.g., HIV, immunosuppression, aging, malnutrition). Unlike primary TB, it classically involves the apical/...

Thromboxane A2 (TXA2) is a prostanoid produced by activated platelets via cyclooxygenase-1 (COX-1) and thromboxane synthase. It promotes platelet aggregation and vasoconstriction, playing a key role in primary hemostasis and arterial thr...

Krukenberg tumor is a metastatic ovarian tumor, classically bilateral, originating from a gastrointestinal (most commonly gastric, especially diffuse-type/signet ring) or breast primary. Histologically characterized by mucin-filled signe...

"Diffuse type" most commonly refers to the diffuse type of gastric adenocarcinoma in the Lauren classification system, characterized by poorly cohesive signet ring cells that infiltrate the stomach wall without forming glands. It carries...

Tyrosine hydroxylase is the rate-limiting enzyme in catecholamine biosynthesis, converting L-tyrosine to L-DOPA using tetrahydrobiopterin (BH4) as a cofactor. It is expressed in the adrenal medulla, sympathetic neurons, and dopaminergic...

Secondary syphilis occurs 4-10 weeks after the primary chancre resolves due to hematogenous and lymphatic dissemination of Treponema pallidum. It classically presents with a diffuse maculopapular rash involving the palms and soles, along...

Tricyclic antidepressants (TCAs) are older antidepressants that block reuptake of serotonin and norepinephrine at presynaptic terminals. They are used for depression, neuropathic pain, migraine prophylaxis, and enuresis, but their use is...

Liver acinus zone 1 (periportal zone) refers to the hepatocytes closest to the portal triad (hepatic artery, portal vein, bile duct), receiving the most oxygenated blood. These cells are the first affected in ischemia-related changes due...

The sinoatrial (SA) node is the heart's primary pacemaker, located at the junction of the right atrium and superior vena cava. It generates spontaneous action potentials at ~60-100 bpm via unstable phase 4 depolarization (funny current,...

MAP (mitogen-activated protein) kinases are a family of serine/threonine kinases that transduce extracellular signals (growth factors, cytokines, stress) into intracellular responses via sequential phosphorylation cascades (MAPKKK→MAPKK→...

22q11 deletion syndrome (DiGeorge syndrome) results from a microdeletion at chromosome 22q11.2, causing a classic triad of congenital heart defects, hypocalcemia (due to hypoplastic parathyroid glands), and immunodeficiency (due to thymi...

Non-dihydropyridine calcium channel blockers (verapamil and diltiazem) preferentially block L-type calcium channels in cardiac tissue (SA/AV node and myocardium) rather than vascular smooth muscle. They are used for rate control in atria...

Trisomy 13, or Patau syndrome, is a chromosomal disorder caused by an extra copy of chromosome 13, resulting in severe congenital anomalies. It is characterized by microcephaly with holoprosencephaly, cleft lip/palate, polydactyly, and c...