Medical terms, defined.

DNA-dependent RNA polymerase is the enzyme responsible for synthesizing RNA from a DNA template, catalyzing transcription in both prokaryotes and eukaryotes. In eukaryotes, three distinct polymerases (I, II, III) transcribe different cla...

Schistocytes are fragmented red blood cells seen on peripheral blood smear, classically appearing as helmet cells or triangular/crescent-shaped fragments. They are a hallmark finding of microangiopathic hemolytic anemia (MAHA) and mechan...

Dysentery is a type of gastroenteritis characterized by bloody diarrhea with mucus, abdominal cramping, and tenesmus, caused by invasive pathogens that damage the colonic mucosa. Classic causes include Shigella, Entamoeba histolytica, Ca...

Microangiopathic hemolytic anemia (MAHA) is a Coombs-negative hemolytic anemia caused by mechanical destruction of red blood cells as they pass through narrowed or damaged small vessels containing fibrin strands or platelet microthrombi....

Hemolytic Uremic Syndrome (HUS) is a triad of microangiopathic hemolytic anemia, thrombocytopenia, and acute kidney injury. It most commonly occurs in children following infection with Shiga toxin-producing E. coli (STEC), particularly s...

Thrombotic thrombocytopenic purpura (TTP) is a life-threatening microangiopathy caused by deficiency or inhibition of ADAMTS13, leading to accumulation of ultra-large vWF multimers and platelet microthrombi. It classically presents with...

Thrombotic Thrombocytopenic Purpura (TTP) is a thrombotic microangiopathy caused by deficiency or inhibition of ADAMTS13, leading to large vWF multimers that cause platelet microthrombi, hemolysis, and organ ischemia. It classically pres...

GPIb (Glycoprotein Ib) is a platelet surface receptor that binds von Willebrand factor (vWF), mediating platelet adhesion to exposed subendothelial collagen at sites of vascular injury. It is the key receptor in primary hemostasis's init...

Ecchymoses are larger (>1 cm) areas of skin discoloration caused by extravasation of blood into subcutaneous tissue, commonly known as bruises. They differ from petechiae (<3mm) and purpura (3mm-1cm) by size, with all three representing...

GPIIb/IIIa (integrin αIIbβ3) is a platelet surface glycoprotein receptor that binds fibrinogen, linking platelets together to form the platelet aggregation plug during primary hemostasis. It is activated following platelet adhesion and A...

The ristocetin cofactor assay (and related ristocetin-induced platelet aggregation, RIPA) measures von Willebrand factor (vWF) function by assessing its ability to bind platelet GPIb receptors in the presence of the antibiotic ristocetin...

GPIb (glycoprotein Ib) is a platelet surface receptor that binds von Willebrand factor (vWF), mediating platelet adhesion to the subendothelial matrix at sites of vascular injury. It is essential for the initial step of primary hemostasi...

vWF metalloprotease, also known as ADAMTS13, cleaves large von Willebrand factor (vWF) multimers into smaller, less active forms. Deficiency or inhibition of ADAMTS13 leads to accumulation of ultra-large vWF multimers, causing excessive...

P2Y12 is a Gi-protein-coupled ADP receptor on platelet surfaces that promotes platelet activation and aggregation upon binding ADP. It is the pharmacologic target of antiplatelet drugs like clopidogrel, prasugrel, and ticagrelor used in...

GPIIb/IIIa (integrin αIIbβ3) is a platelet surface receptor that binds fibrinogen and von Willebrand factor, mediating platelet-platelet aggregation—the final common pathway of primary hemostasis. Congenital deficiency causes Glanzmann t...

Dactylitis, also called 'sausage digit,' refers to diffuse swelling of an entire finger or toe due to inflammation of the digit including the tendons, joints, and soft tissue. It is a hallmark finding of seronegative spondyloarthropathie...

Enteritis refers to inflammation of the small intestine, commonly caused by infectious agents (viral, bacterial, or parasitic) and presenting with diarrhea, abdominal pain, nausea, and vomiting. It is often used alongside 'gastroenteriti...

Sprue refers to malabsorptive disorders causing chronic diarrhea, steatorrhea, and nutrient deficiencies due to small bowel mucosal damage. The two main types are celiac sprue (gluten-sensitive enteropathy) and tropical sprue (associated...

Trendelenburg gait is an abnormal, waddling gait pattern caused by weakness of the hip abductor muscles (gluteus medius and minimus), resulting in pelvic drop on the contralateral side during the stance phase of the affected limb. It is...

The Trendelenburg sign is a clinical finding indicating weakness of the hip abductor muscles (gluteus medius and minimus) or superior gluteal nerve dysfunction. When standing on the affected leg, the contralateral pelvis drops instead of...

The sartorius is the longest muscle in the body, a thin, strap-like muscle that runs obliquely across the anterior thigh from the anterior superior iliac spine (ASIS) to the medial proximal tibia. It flexes, abducts, and laterally rotate...

The pectineus is a flat, quadrangular muscle in the anteromedial thigh that acts as both a hip flexor and adductor. It belongs to the medial (adductor) compartment but is unique in receiving dual innervation from both the femoral and obt...

Anucleate refers to a cell that lacks a nucleus. Key examples in humans include mature red blood cells (erythrocytes) and the cells of the lens (lens fibers), which extrude their nuclei during terminal differentiation to optimize function.

Leishmania is a genus of protozoan parasites transmitted by the bite of female sandflies (Phlebotomus in Old World, Lutzomyia in New World), causing cutaneous, mucocutaneous, or visceral leishmaniasis. Visceral leishmaniasis (kala-azar),...

Megakaryocytes are large, polyploid bone marrow cells responsible for producing platelets (thrombocytes) through cytoplasmic fragmentation. They are regulated primarily by thrombopoietin (TPO), produced mainly in the liver. Each megakary...

Megakaryocytes are large, polyploid bone marrow cells responsible for platelet production. Each megakaryocyte can produce thousands of platelets via cytoplasmic fragmentation. They are derived from hematopoietic stem cells under the infl...

Shiga toxin (Stx) is an AB5 exotoxin produced by Shigella dysenteriae type 1 and Shiga toxin-producing E. coli (STEC, e.g., O157:H7, which makes Stx1/Stx2). It cleaves the 28S rRNA of the 60S ribosomal subunit, which inhibits protein syn...

Plasmids are extrachromosomal, circular, double-stranded DNA molecules in bacteria that replicate independently of the chromosome and often carry nonessential genes such as antibiotic resistance, toxins, and virulence factors. They sprea...

Diphtheria toxin is an A-B exotoxin produced by lysogenic Corynebacterium diphtheriae (carrying the beta-prophage with the tox gene). It inactivates elongation factor-2 (EF-2) by ADP-ribosylation, halting host protein synthesis and causi...

Fructokinase (ketohexokinase) is the hepatic, renal, and intestinal enzyme that phosphorylates fructose to fructose-1-phosphate using ATP, trapping it in the cell. Deficiency causes essential fructosuria, a benign, asymptomatic autosomal...

Protein S is a vitamin K-dependent anticoagulant protein synthesized in the liver (also by endothelium and megakaryocytes). It acts as a cofactor for activated protein C (APC) to inactivate factors Va and VIIIa. Deficiency (usually autos...

PYR (pyrrolidonyl arylamidase / L-pyrrolidonyl-β-naphthylamide hydrolysis test) is a rapid biochemical test that detects the enzyme pyrrolidonyl arylamidase. It is positive in Group A Streptococcus (S. pyogenes) and Enterococcus, and it...

IL-6 is a pleiotropic cytokine secreted by macrophages, T cells, and endothelial cells that mediates the acute-phase response and fever. It stimulates hepatic production of acute-phase reactants (CRP, fibrinogen, hepcidin, ferritin) and...

Thrombomodulin is an endothelial cell surface receptor that binds thrombin, converting it from a procoagulant enzyme into an anticoagulant that activates protein C. Activated protein C (with cofactor protein S) inactivates factors Va and...

Sleeve gastrectomy is a bariatric procedure that removes ~75–80% of the stomach (greater curvature, including the fundus), leaving a narrow tubular "sleeve." It works through restriction plus hormonal changes (markedly decreased ghrelin)...

Dalteparin (Fragmin) is a low-molecular-weight heparin (LMWH) that potentiates antithrombin III, primarily inhibiting factor Xa (greater anti-Xa than anti-IIa activity). It is used for DVT/PE prophylaxis and treatment, unstable angina/NS...

Vitamin B12 (cobalamin) is a water-soluble vitamin required for DNA synthesis (via methionine synthase) and myelin maintenance (via methylmalonyl-CoA mutase). Deficiency causes megaloblastic anemia with hypersegmented neutrophils, elevat...

Phospholipase A2 (PLA2) is an enzyme that hydrolyzes the sn-2 ester bond of membrane phospholipids, releasing arachidonic acid (the precursor of prostaglandins, thromboxanes, leukotrienes) and lysophospholipid. It is inhibited indirectly...

Macrocytic describes red blood cells larger than normal (MCV >100 fL). Macrocytic anemias are divided into megaloblastic (impaired DNA synthesis: B12/folate deficiency; hypersegmented neutrophils) and non-megaloblastic (alcohol, liver di...

Botulinum toxin is a neurotoxin produced by Clostridium botulinum (gram-positive, spore-forming, obligate anaerobic rod) that cleaves SNARE proteins and blocks presynaptic acetylcholine release at the neuromuscular junction. This causes...

The secretin stimulation test has two classic uses. (1) Gastrinoma (Zollinger-Ellison syndrome): IV secretin paradoxically RAISES serum gastrin (a rise of ≥120 pg/mL is positive), whereas normal G cells and antral G-cell hyperplasia show...

Cholera toxin is an AB5 enterotoxin from Vibrio cholerae that ADP-ribosylates the Gs alpha subunit, permanently activating adenylyl cyclase. The resulting rise in cAMP opens CFTR in intestinal crypt cells, causing massive Cl-/water efflu...

Vitamin B12 (cobalamin) is a water-soluble vitamin required for DNA synthesis (methionine synthase) and odd-chain fatty acid/amino acid metabolism (methylmalonyl-CoA mutase). Deficiency causes megaloblastic anemia plus neurologic finding...

Roux-en-Y is a surgical reconstruction in which a transected segment of small bowel (the Roux limb) is anastomosed to another structure (stomach pouch, esophagus, or bile duct), and the proximal limb (biliopancreatic limb) is joined to t...

Gram-negative sepsis is a life-threatening systemic response to bloodstream infection with gram-negative bacilli (e.g., E. coli, Klebsiella, Pseudomonas), driven largely by LPS (endotoxin, lipid A) activating TLR4 on macrophages. It caus...

Giardia lamblia (G. duodenalis/intestinalis) is a flagellated protozoan that causes giardiasis: foul-smelling, greasy, non-bloody watery diarrhea with malabsorption, bloating, and flatulence. It is acquired by ingesting cysts in contamin...

Paragonimus westermani is the oriental (Asian) lung fluke, a trematode acquired by eating raw or undercooked freshwater crabs or crayfish. It causes pulmonary paragonimiasis: chronic cough, hemoptysis (rusty/"chocolate" sputum), and cavi...

Bismuth subsalicylate (Pepto-Bismol) is a heavy-metal-based agent used for traveler's diarrhea, dyspepsia, and as a component of bismuth quadruple therapy for H. pylori. It has antimicrobial, antisecretory, and anti-inflammatory effects,...

Behçet disease is a chronic, relapsing variable-vessel vasculitis characterized by recurrent painful oral aphthous ulcers, genital ulcers, and uveitis. It is associated with HLA-B51, is most common along the historic Silk Road (Turkey, M...

Vanishing bile duct syndrome (VBDS) is an acquired condition characterized by progressive destruction and loss of intrahepatic bile ducts (ductopenia, defined as bile ducts missing in >50% of portal tracts), leading to cholestasis and po...