Medical terms, defined.

Medium-chain acyl-CoA dehydrogenase (MCAD) is a mitochondrial enzyme catalyzing the first step of beta-oxidation of C6–C12 fatty acids. MCAD deficiency (autosomal recessive, ACADM gene, most commonly the K304E mutation) is the most commo...

Acetone is the simplest ketone body (3-carbon), formed by spontaneous decarboxylation of acetoacetate during ketosis (DKA, starvation, alcoholic ketoacidosis). It is volatile and causes the "fruity" breath odor of DKA; it is NOT detected...

Beta-hydroxybutyrate (BHB) is the most abundant ketone body (~75% in DKA), produced in hepatic mitochondria from acetyl-CoA during fasting, prolonged exercise, low-carb diets, alcoholic ketoacidosis, and DKA. It is a major alternative fu...

Biotin carboxylase is the enzymatic domain/subunit (in bacterial acetyl-CoA carboxylase) or the ATP-dependent carboxylation step (in human biotin-dependent carboxylases) that attaches CO2 (from bicarbonate) to the biotin cofactor, formin...

Dihydrobiopterin (BH2) is the oxidized form of the cofactor tetrahydrobiopterin (BH4), which is required by phenylalanine, tyrosine, and tryptophan hydroxylases, as well as nitric oxide synthase. BH2 is regenerated to BH4 by dihydropteri...

MAO (monoamine oxidase) is a mitochondrial enzyme that oxidatively deaminates monoamine neurotransmitters (norepinephrine, serotonin, dopamine) and dietary amines like tyramine. MAO-A prefers serotonin/NE/tyramine; MAO-B prefers dopamine...

BH4 (tetrahydrobiopterin) is an essential cofactor for phenylalanine hydroxylase, tyrosine hydroxylase, tryptophan hydroxylase, and nitric oxide synthase. Deficiency (impaired synthesis or recycling) causes "malignant" hyperphenylalanine...

S-adenosylmethionine (SAM/SAMe) is the universal methyl donor in the body, synthesized from methionine + ATP by methionine adenosyltransferase. After donating its methyl group it becomes S-adenosylhomocysteine (SAH), then homocysteine, w...

Dopamine beta-hydroxylase (DBH) is a copper-containing enzyme that converts dopamine to norepinephrine inside synaptic vesicles of noradrenergic neurons and adrenal medullary chromaffin cells. It requires copper (Cu2+) and vitamin C (asc...

Orotate (orotic acid) is an intermediate in de novo pyrimidine synthesis, formed from dihydroorotate by dihydroorotate dehydrogenase and converted to OMP by UMP synthase (orotate phosphoribosyltransferase). Accumulates and is excreted in...

Carbamoyl phosphate synthetase I (CPS I) is the mitochondrial, rate-limiting enzyme of the urea cycle. It combines NH3 (ammonia), HCO3-, and 2 ATP to form carbamoyl phosphate, and it requires N-acetylglutamate (NAG) as an obligate allost...

N-acetylglutamate (NAG) is the obligate allosteric activator of carbamoyl phosphate synthetase I (CPS I), the rate-limiting enzyme of the urea cycle in liver mitochondria. It is synthesized from glutamate + acetyl-CoA by NAG synthase, wh...

CPS II (carbamoyl phosphate synthetase II) is the cytosolic, rate-limiting enzyme of de novo pyrimidine synthesis. It uses glutamine + CO2 + 2 ATP to make carbamoyl phosphate; it is activated by PRPP and ATP and inhibited by UTP. It is d...

Amidophosphoribosyltransferase (glutamine-PRPP amidotransferase, PPAT/GPAT) catalyzes the committed, rate-limiting step of de novo purine synthesis: PRPP + glutamine → 5-phosphoribosylamine + glutamate + PPi. It is inhibited by end-produ...

Catechol-O-methyltransferase (COMT) is an enzyme that degrades catecholamines (dopamine, norepinephrine, epinephrine) and levodopa by transferring a methyl group from S-adenosylmethionine (SAM) to a catechol hydroxyl group. Products incl...

IMP (inosine monophosphate) is the first purine nucleotide formed in de novo purine synthesis and the branch-point precursor of AMP and GMP. It is built on a PRPP backbone, and its synthesis is blocked by drugs such as methotrexate and m...

Inosine monophosphate (IMP) is the first fully formed purine nucleotide, the branch-point precursor for both AMP and GMP. It is made de novo from PRPP via the rate-limiting step (PRPP amidotransferase) and a series of steps requiring gly...

ANP (atrial natriuretic peptide) is a 28-amino-acid peptide hormone released by atrial myocytes in response to atrial stretch (volume expansion). It promotes natriuresis, diuresis, and vasodilation, lowering blood pressure and preload vi...

T3 (triiodothyronine) is the active thyroid hormone, ~80% produced by peripheral conversion of T4 via 5'-deiodinase, with the thyroid secreting the rest. It binds nuclear thyroid hormone receptors with ~10x greater affinity than T4 and r...

Endothelin-1 (ET-1) is the most potent endogenous vasoconstrictor peptide (21 amino acids), secreted mainly by vascular endothelial cells. It acts via ET-A (vasoconstriction, proliferation) and ET-B receptors (endothelial: vasodilation v...

T4 (thyroxine) is the main hormone secreted by the thyroid follicular cells (~90% of output). It is a prohormone with lower potency than T3. It is converted peripherally to active T3 by 5'-deiodinase, and it is >99% bound to carrier prot...

BNP (B-type natriuretic peptide) is a neurohormone released by ventricular myocytes in response to wall stretch and volume/pressure overload. It is elevated in heart failure and is used to differentiate cardiac from pulmonary causes of d...

Adiponectin is an adipokine secreted by adipocytes that enhances insulin sensitivity, stimulates fatty acid oxidation (via AMPK activation), and has anti-inflammatory and anti-atherogenic effects. Unlike most adipokines, its levels are D...

Erythropoietin (EPO) is a glycoprotein hormone produced mainly by peritubular interstitial fibroblast-like cells of the renal cortex in response to hypoxia. It stimulates erythroid progenitor (CFU-E) proliferation and differentiation in...

hPL (human placental lactogen, also called human chorionic somatomammotropin) is a placental peptide hormone that raises maternal insulin resistance and shifts maternal metabolism toward lipolysis and free fatty acid use, sparing glucose...

PTH (parathyroid hormone) is an 84-amino-acid peptide secreted by chief cells of the parathyroid glands in response to low serum ionized Ca2+ (also high phosphate or low 1,25-(OH)2 vitamin D). It raises serum calcium and lowers serum pho...

IGF-1 (somatomedin C) is a peptide hormone produced mainly by the liver in response to growth hormone (GH) binding to the GH receptor (JAK2/STAT5 pathway). It mediates most of GH's growth-promoting effects and exerts negative feedback on...

Activin is a dimeric glycoprotein of the TGF-β superfamily (homo- or heterodimer of inhibin β subunits) that stimulates FSH secretion from anterior pituitary gonadotrophs. It is antagonized by inhibin (α + β subunit) and follistatin, and...

TPO most commonly means thyroid peroxidase, the enzyme that oxidizes iodide, iodinates tyrosine residues on thyroglobulin (organification), and couples MIT and DIT to form T3 and T4. Anti-TPO antibodies are the hallmark of Hashimoto thyr...

PRL (prolactin) is a peptide hormone secreted by lactotrophs of the anterior pituitary. It stimulates milk production and is uniquely under tonic INHIBITORY control by hypothalamic dopamine; TRH and estrogen stimulate it. Hyperprolactine...

hCG (human chorionic gonadotropin) is a glycoprotein hormone (α and β subunits) produced by syncytiotrophoblast cells of the placenta. It maintains the corpus luteum's progesterone production in early pregnancy and is the basis for pregn...

CBG most commonly means corticosteroid-binding globulin (transcortin), a liver-synthesized glycoprotein that carries about 80-90% of circulating cortisol. Only free cortisol (about 5%) is biologically active. Estrogen raises CBG, which r...

Corticosteroid-binding globulin (CBG, transcortin) is a hepatically synthesized glycoprotein (SERPINA6, serpin family) that binds ~80–90% of circulating cortisol with high affinity, ~5–10% is albumin-bound, and only ~5–10% is free (biolo...

TBG (thyroxine-binding globulin) is the main hepatically synthesized plasma carrier protein for thyroid hormones, binding ~70% of circulating T4 and T3. Changes in TBG alter TOTAL T4/T3 but not FREE hormone levels, so patients remain eut...

AIN (acute interstitial nephritis) is an immune-mediated, type IV (delayed) hypersensitivity reaction causing inflammation of the renal interstitium and tubules, most often triggered by drugs (beta-lactams, NSAIDs, PPIs, sulfonamides, ri...

ATN (acute tubular necrosis) is the most common cause of intrinsic acute kidney injury in hospitalized patients, caused by ischemic or nephrotoxic injury to renal tubular epithelial cells. Classic findings are muddy brown granular casts,...

RTA (renal tubular acidosis) is a group of disorders with a normal anion gap (hyperchloremic) metabolic acidosis due to impaired renal bicarbonate reabsorption (type 2) or impaired H+ secretion/ammonium excretion (types 1 and 4). Key typ...

A staghorn calculus is a large renal stone that branches to fill the renal pelvis and 2 or more calyces, which gives it an antler-like shape. It is classically made of struvite (magnesium ammonium phosphate) and forms in alkaline urine i...

The canal of Schlemm (scleral venous sinus) is a circumferential, endothelium-lined channel at the corneoscleral limbus that drains aqueous humor from the anterior chamber via the trabecular meshwork into the episcleral venous system. Im...

The trabecular meshwork is a sponge-like network of connective tissue and endothelial-lined beams in the iridocorneal angle that drains ~90% of aqueous humor (conventional outflow) into Schlemm's canal. Impaired outflow through it raises...

Urolithiasis is the formation of stones (calculi) anywhere in the urinary tract, most commonly calcium oxalate stones. It classically presents with acute, colicky flank pain radiating to the groin, with hematuria. Noncontrast CT is the d...

The ciliary muscle is a smooth muscle ring in the ciliary body of the eye that controls lens shape for accommodation (near vision). It is innervated by parasympathetic fibers (CN III, via the Edinger-Westphal nucleus → ciliary ganglion →...

The optic tract is the continuation of the visual pathway posterior to the optic chiasm, carrying contralateral visual field information (ipsilateral temporal retina + contralateral nasal retina fibers) to the lateral geniculate nucleus...

The macula lutea is a yellowish, oval region of the central posterior retina, lateral to the optic disc, that is specialized for high-acuity central vision. Its center, the fovea centralis, contains only cones and has the highest visual...

The fovea centralis is a small central depression in the macula lutea of the retina that provides the highest visual acuity. It contains only cones (no rods), and the inner retinal layers are displaced laterally, so light reaches the pho...

The ciliary body is the middle portion of the uvea (between iris and choroid) that produces aqueous humor (ciliary epithelium) and controls lens accommodation (ciliary muscle). Parasympathetic input (CN III via the ciliary ganglion, musc...

Age-related macular degeneration (AMD) is the leading cause of irreversible central vision loss in people over 50 in developed countries. It causes degeneration of the macula (central retina), with drusen deposits in dry (atrophic, ~90%)...

The iris is the pigmented, contractile diaphragm of the anterior eye that surrounds the pupil and controls the amount of light reaching the retina. It contains the sphincter pupillae (parasympathetic, CN III, M3 receptors → miosis) and d...

Central retinal vein occlusion (CRVO) is thrombotic blockage of the central retinal vein, typically at or posterior to the lamina cribrosa. It causes sudden, painless, unilateral vision loss. Fundoscopy shows the classic "blood and thund...

Central retinal artery occlusion (CRAO) is an ophthalmologic emergency causing sudden, painless, monocular vision loss, usually from embolism (carotid atheroma or cardiac source). Fundoscopy shows a pale, whitened retina with a cherry-re...