Medical terms, defined.

Virchow node is an enlarged, hard, palpable left supraclavicular lymph node, classically signaling metastatic abdominal (especially gastric) malignancy. It is the nodal finding of Troisier sign.

CEA (carcinoembryonic antigen) is an oncofetal glycoprotein tumor marker, most associated with colorectal adenocarcinoma. It is used to monitor treatment response and detect recurrence after resection, NOT to screen or diagnose, because...

CA-125 is a glycoprotein (MUC16) tumor marker most associated with epithelial ovarian cancer (serous type). It is used to monitor treatment response and detect recurrence, and to help assess pelvic masses, but it is NOT a screening test...

Carcinoembryonic antigen (CEA) is an oncofetal glycoprotein (cell-adhesion molecule, CEACAM5) normally produced in fetal gut and at low levels in adults. It is a tumor marker most associated with colorectal adenocarcinoma, used for monit...

The Jones criteria are the diagnostic criteria for acute rheumatic fever (ARF), a post-infectious, immune-mediated sequela of Group A Streptococcus (S. pyogenes) pharyngitis. Diagnosis requires evidence of a preceding GAS infection plus...

Alpha-fetoprotein (AFP) is a glycoprotein made by the fetal yolk sac, liver, and GI tract. In adults it serves as a tumor marker for hepatocellular carcinoma and yolk sac (endodermal sinus) tumors. In pregnancy, maternal serum AFP is a s...

PSA (prostate-specific antigen) is a serine protease (kallikrein-3, KLK3) produced by prostatic epithelial cells that liquefies semen by cleaving semenogelin. Serum PSA is organ-specific but NOT cancer-specific; it's elevated in prostate...

S-100 is a calcium-binding protein found in cells derived from the neural crest (melanocytes, Schwann cells), plus astrocytes, chondrocytes, adipocytes, and Langerhans cells. It is a high-yield immunohistochemical marker for melanoma, sc...

Cytokeratins are intermediate filament proteins (keratins) found in epithelial cells that form the cytoskeleton and provide mechanical strength. Because they persist in epithelial-derived tumors, cytokeratin immunohistochemistry (e.g., C...

Ki-67 is a nuclear protein expressed in all active phases of the cell cycle (G1, S, G2, M) but absent in quiescent G0 cells. It is detected by immunohistochemistry (MIB-1 antibody) and the Ki-67 labeling index serves as a marker of tumor...

Beta-hCG (human chorionic gonadotropin, β-subunit) is a glycoprotein hormone produced by syncytiotrophoblast cells of the placenta. It maintains the corpus luteum (and progesterone production) in early pregnancy, and is the basis of urin...

Coenzyme Q10 (CoQ10, ubiquinone) is a lipid-soluble electron carrier in the mitochondrial electron transport chain that shuttles electrons from Complexes I and II to Complex III. It also functions as an antioxidant. High-yield clinical l...

ETC = Electron Transport Chain: a series of 4 protein complexes (I-IV) in the inner mitochondrial membrane that transfer electrons from NADH and FADH2 to O2, pumping H+ into the intermembrane space. The resulting proton gradient drives A...

Complex V (ATP synthase, F0F1 ATP synthase) is the final enzyme complex of oxidative phosphorylation in the inner mitochondrial membrane. It uses the proton gradient created by complexes I, III, and IV to phosphorylate ADP into ATP (~1 A...

The cytochrome bc1 complex (Complex III, ubiquinol-cytochrome c oxidoreductase) is an inner mitochondrial membrane enzyme of the electron transport chain. It transfers electrons from ubiquinol (CoQH2) to cytochrome c and pumps protons in...

Cytochrome b/c1 is Complex III (ubiquinol-cytochrome c oxidoreductase, the bc1 complex) of the mitochondrial electron transport chain. It transfers electrons from ubiquinol (CoQH2) to cytochrome c and pumps protons into the intermembrane...

An anal fistula (fistula-in-ano) is an abnormal epithelialized tract connecting the anal canal (at the dentate line, from an infected anal crypt gland) to the perianal skin. It usually results from a previous perianal abscess and present...

Immune-complex mediated glomerulonephritis (GN) is glomerular injury caused by deposition (or in situ formation) of antigen-antibody complexes in the glomerulus, activating complement and recruiting inflammatory cells. It is a type III h...

HSP most commonly = Henoch-Schönlein purpura (now IgA vasculitis), the most common childhood systemic vasculitis. It is an IgA immune complex-mediated small-vessel leukocytoclastic vasculitis causing the tetrad of palpable purpura (butto...

The vitreous humor is a clear, avascular, gel-like substance filling the posterior segment of the eye (between the lens and retina), composed of ~99% water plus collagen type II and hyaluronic acid. It maintains eye shape, holds the reti...

Immune-complex mediated glomerulonephritides are glomerular diseases caused by deposition (or in situ formation) of antigen-antibody complexes in the glomerulus (type III hypersensitivity). This activates complement and recruits inflamma...

Angina pectoris is substernal chest pain/pressure caused by transient myocardial ischemia from a supply-demand mismatch, without myocyte necrosis (no troponin elevation). Classic cause is fixed atherosclerotic obstruction (≥70%) of coron...

Angle-closure glaucoma is an ophthalmologic emergency in which the iris physically blocks the trabecular meshwork (pupillary block), causing a sudden rise in intraocular pressure (IOP) and optic nerve damage. It presents with severe eye...

Emphysematous cholecystitis is a rare, severe form of acute cholecystitis in which gas-forming bacteria (Clostridium perfringens, E. coli, Klebsiella) produce gas in the gallbladder lumen, wall, or pericholecystic tissues. It classically...

DHT (dihydrotestosterone) is a potent androgen made from testosterone by 5-alpha-reductase. It drives male external genitalia development in utero and, later in life, prostate growth, male-pattern baldness, and acne. 5-alpha-reductase in...

Ubiquinol is the fully reduced form (QH2, hydroquinone) of coenzyme Q10 (ubiquinone). It carries electrons from Complexes I and II to Complex III in the mitochondrial electron transport chain and acts as a lipid-soluble antioxidant in me...

5α-reductase is an enzyme that converts testosterone to dihydrotestosterone (DHT), a more potent androgen that drives development of male external genitalia, prostate, and androgenic hair/scalp effects. Deficiency causes 46,XY DSD with a...

Icterus is the medical term for jaundice: yellow discoloration of the skin, sclera, and mucous membranes caused by hyperbilirubinemia. It becomes clinically visible when total serum bilirubin exceeds about 2-3 mg/dL, and scleral icterus...

The chemiosmotic hypothesis (Peter Mitchell, Nobel 1978) states that the energy released by electron transport through the electron transport chain (ETC) is used to pump protons (H+) across the inner mitochondrial membrane, creating an e...

F0F1 ATP synthase (Complex V) is the inner mitochondrial membrane enzyme that uses the proton gradient created by the electron transport chain to phosphorylate ADP into ATP (oxidative phosphorylation). F0 is the membrane-embedded proton...

Iron-sulfur (Fe-S) clusters are prosthetic groups made of iron and inorganic sulfide (e.g., [2Fe-2S], [4Fe-4S]) that are bound to proteins by cysteine residues. They act as one-electron carriers in redox reactions, most notably in Comple...

5-alpha reductase is an enzyme that converts testosterone to dihydrotestosterone (DHT), a more potent androgen that binds the androgen receptor with higher affinity. DHT drives development of male external genitalia, the prostate, and ma...

UCP1 (uncoupling protein 1, thermogenin) is an inner mitochondrial membrane proton channel found in brown adipose tissue. It dissipates the proton gradient as heat instead of ATP synthesis, uncoupling oxidative phosphorylation to produce...

Uncoupling proteins (UCPs, e.g., UCP1/thermogenin) are inner mitochondrial membrane proteins that let protons leak back into the matrix, bypassing ATP synthase. This dissipates the proton gradient as heat instead of making ATP, which dri...

Proton motive force (PMF) is the electrochemical gradient of H+ across the inner mitochondrial membrane (or bacterial plasma membrane), created by the electron transport chain pumping protons out of the matrix. It has two components: a c...

2,4-Dinitrophenol (DNP) is a lipophilic weak acid that acts as a mitochondrial protonophore, uncoupling oxidative phosphorylation: the proton gradient dissipates as heat instead of driving ATP synthase. It causes hyperthermia, tachycardi...

Glycerol kinase (GK) is a liver/kidney enzyme that phosphorylates glycerol to glycerol-3-phosphate using ATP, allowing glycerol from lipolysis to enter gluconeogenesis or triglyceride synthesis. Adipose tissue lacks GK, so it cannot reus...

The P/O ratio is the number of ATP molecules synthesized per atom of oxygen reduced (i.e., per pair of electrons transferred through the electron transport chain). Classic values: ~2.5 for NADH and ~1.5 for FADH2 (older textbooks: 3 and...

Phosphoenolpyruvate carboxykinase (PEPCK) is a rate-limiting gluconeogenic enzyme that converts oxaloacetate to phosphoenolpyruvate (PEP) using GTP, releasing CO2. It is induced by glucagon, cortisol, and epinephrine, and suppressed by i...

Substrate-level phosphorylation is the direct transfer of a phosphate group from a high-energy substrate (e.g., 1,3-bisphosphoglycerate, phosphoenolpyruvate, succinyl-CoA) to ADP/GDP to form ATP/GTP, without involving the electron transp...

Oxidative decarboxylation is a reaction in which a substrate loses CO2 (decarboxylation) while simultaneously being oxidized, with electrons transferred to NAD+ (or another acceptor) to form NADH. The high-yield examples are the pyruvate...

CPT-I (carnitine palmitoyltransferase I, also called CAT-I) is the rate-limiting enzyme of fatty acid β-oxidation. It sits on the outer mitochondrial membrane and converts long-chain acyl-CoA to acylcarnitine so it can enter the mitochon...

Carnitine palmitoyltransferase I (CPT-I, also called CPT1) is the rate-limiting enzyme of the carnitine shuttle. It sits on the outer mitochondrial membrane and converts long-chain fatty acyl-CoA to acylcarnitine so fatty acids can enter...

Acyl-CoA dehydrogenases are FAD-dependent mitochondrial enzymes that catalyze the first step of each cycle of fatty acid β-oxidation, forming a trans-Δ2-enoyl-CoA from an acyl-CoA. Isoforms are chain-length specific (SCAD, MCAD, LCAD, VL...

Carnitine palmitoyltransferase II (CPT II) is an enzyme on the inner mitochondrial membrane (matrix side) that converts acylcarnitine back to acyl-CoA, allowing long-chain fatty acids to undergo beta-oxidation. Deficiency (AR) is the mos...

MCAD (medium-chain acyl-CoA dehydrogenase) deficiency is the most common inherited fatty acid oxidation disorder (autosomal recessive, ACADM gene). It presents in infancy/early childhood with fasting-induced hypoketotic hypoglycemia, vom...

CPT-II (carnitine palmitoyltransferase II) is an inner mitochondrial membrane enzyme that converts acyl-carnitine back to acyl-CoA so long-chain fatty acids can enter beta-oxidation. Deficiency is the most common inherited disorder of li...

Thiolase (acetyl-CoA acetyltransferase; β-ketothiolase) catalyzes the reversible thiolytic cleavage of acetoacetyl-CoA (or 3-ketoacyl-CoA) into acetyl-CoA using CoA. It performs the final step of each β-oxidation cycle and the first step...

Ketothiolase (mitochondrial acetoacetyl-CoA thiolase, T2; encoded by ACAT1) catalyzes the reversible thiolytic cleavage of 2-methylacetoacetyl-CoA and acetoacetyl-CoA, and it is needed for isoleucine catabolism and ketone body utilizatio...

HMG-CoA lyase is a mitochondrial enzyme that cleaves HMG-CoA into acetyl-CoA and acetoacetate, the final step of ketogenesis and of leucine catabolism. Deficiency is an autosomal recessive disorder causing fasting hypoketotic hypoglycemi...