1500+ medical terms, defined.

Yersinia pestis is a gram-negative rod with bipolar 'safety pin' staining that causes plague, transmitted by the rat flea. It produces bubonic, septicaemic, and pneumonic forms, the last spreading person to person by respiratory droplets.

Polioviruses are the three serotypes of an enterovirus in the family Picornaviridae, non-enveloped with a positive-sense single-stranded RNA genome. They are acid-stable, allowing passage through the stomach, and are tropic for anterior...

Echoviruses are non-enveloped, positive-sense single-stranded RNA enteroviruses of the family Picornaviridae. They are a leading cause of aseptic meningitis, particularly in children during summer and early autumn.

Picornaviridae are small, non-enveloped viruses with a positive-sense single-stranded RNA genome. The family includes poliovirus, coxsackievirus, echovirus, rhinovirus, and hepatitis A virus.

Herpes simplex virus type 2 is an enveloped double-stranded DNA herpesvirus that causes most genital herpes and establishes lifelong latency in sacral dorsal root ganglia. It is a leading cause of neonatal herpes when acquired during vag...

Berger disease, also known as IgA nephropathy, is the most common cause of primary glomerulonephritis worldwide. It classically presents with episodic gross or microscopic hematuria occurring 1-3 days after an upper respiratory or GI inf...

The posterior pituitary (neurohypophysis) is a neural extension of the hypothalamus that stores and secretes two hormones—antidiuretic hormone (ADH/vasopressin) and oxytocin—synthesized by hypothalamic neurons. Unlike the anterior pituit...

Central diabetes insipidus (CDI) is caused by insufficient secretion of antidiuretic hormone (ADH/vasopressin) from the posterior pituitary/hypothalamus, leading to impaired renal water reabsorption. This results in polyuria with dilute...

Aspartic acid (Asp, D) is a negatively charged (acidic) nonessential amino acid that functions as an excitatory neurotransmitter and a key participant in the urea cycle and purine/pyrimidine synthesis. It also serves as a nitrogen donor...

Primary (hereditary) hemochromatosis is an autosomal recessive disorder, most commonly caused by mutations in the HFE gene (C282Y homozygosity), leading to inappropriately low hepcidin levels and increased intestinal iron absorption. Thi...

Opioid use disorder (OUD) is a chronic, relapsing condition characterized by compulsive opioid use despite harmful consequences, tolerance, withdrawal, and loss of control over use. It is diagnosed using DSM-5 criteria requiring at least...

Conn syndrome is primary hyperaldosteronism, most commonly caused by an aldosterone-producing adrenal adenoma (or bilateral adrenal hyperplasia). It classically presents with hypertension, hypokalemia, and metabolic alkalosis due to exce...

The penile (spongy) urethra is the longest segment of the male urethra, extending from the bulbous urethra through the corpus spongiosum to the external urethral meatus at the glans penis. It is lined predominantly by pseudostratified/st...

The sarcoplasmic reticulum (SR) is a specialized form of smooth endoplasmic reticulum found in skeletal and cardiac muscle cells that stores and releases calcium ions to regulate muscle contraction and relaxation. It plays a central role...

C-reactive protein (CRP) is an acute-phase reactant synthesized by the liver in response to IL-6 during inflammation, infection, or tissue damage. It is a nonspecific marker used clinically to detect and monitor inflammatory states, infe...

Paraneoplastic syndromes are clinical manifestations caused indirectly by a tumor, mediated by ectopic hormone production, autoimmune cross-reactivity, or cytokine release, rather than by direct tumor invasion or metastasis. They often p...

The marginal zone is the outer region of splenic white pulp surrounding the periarteriolar lymphoid sheath (PALS) and follicles, containing specialized B cells that respond to blood-borne antigens. It's a site of extramedullary hematopoi...

Caseous necrosis is a distinctive form of coagulative necrosis with a cheese-like, friable white-tan gross appearance, classically caused by Mycobacterium tuberculosis and certain fungal infections. Microscopically, it shows amorphous gr...

Tocopherol is the primary form of vitamin E, a fat-soluble vitamin that functions as an antioxidant, protecting cell membranes from oxidative damage by scavenging free radicals. Deficiency is rare but can cause hemolytic anemia (especial...

Vitamin E (alpha-tocopherol) is a fat-soluble vitamin that acts as an antioxidant, protecting cell membranes from free radical damage by scavenging reactive oxygen species. Deficiency is rare but classically causes hemolytic anemia (in p...

Electrical alternans is an ECG finding characterized by beat-to-beat variation in the amplitude or axis of the QRS complex (and sometimes P and T waves), classically associated with large pericardial effusion and cardiac tamponade. It re...

Humoral immunity is the arm of adaptive immunity mediated by B lymphocytes and their secreted antibodies, which target extracellular pathogens (bacteria, viruses in extracellular phase) and toxins. It involves B cell activation, differen...

Autosomal dominant polycystic kidney disease (ADPKD) is a genetic disorder characterized by progressive development of multiple bilateral renal cysts, leading to kidney enlargement and eventual end-stage renal disease (ESRD), typically p...

Uric acid stones are radiolucent kidney stones that form when urine is persistently acidic (pH <5.5) and/or uric acid concentration is high, often seen in gout, high-purine diets, malignancy (tumor lysis syndrome), and metabolic syndrome...

Digitalis refers to cardiac glycosides (e.g., digoxin) derived from the foxglove plant, used primarily for heart failure with reduced ejection fraction and rate control in atrial fibrillation. It works by inhibiting the Na+/K+-ATPase pum...

UTIs are infections of the urinary tract, most commonly caused by ascending bacterial colonization from the fecal flora (E. coli >80%). Classified as cystitis (bladder), pyelonephritis (kidney), or urethritis, presenting with dysuria, fr...

Exudate is a protein-rich, cellular fluid that leaks out of blood vessels into tissue or body cavities as a result of increased vascular permeability, typically due to inflammation or infection. It has high protein content (>3 g/dL) and...

Exotoxins are potent, secreted protein toxins produced by both Gram-positive and Gram-negative bacteria that cause disease by damaging host cells or disrupting cellular processes. They are highly antigenic, often with high toxicity even...

Abdominal X-ray (KUB - Kidney, Ureter, Bladder) is a plain radiograph used as a first-line imaging modality to evaluate the abdomen, primarily for detecting bowel obstruction, perforation (free air), foreign bodies, and certain calcifica...

Active transport is the movement of molecules across a cell membrane against their concentration or electrochemical gradient, requiring energy input (usually ATP). It is essential for maintaining cellular homeostasis, ion gradients, and...

Sodium (Na+) is the primary extracellular cation, critical for maintaining plasma osmolality, fluid balance, and neuromuscular function. Normal serum concentration is 135-145 mEq/L. Disorders of sodium (hyponatremia/hypernatremia) primar...

Neonatal jaundice is yellowish discoloration of the skin and sclera in newborns due to elevated unconjugated (or sometimes conjugated) bilirubin, occurring in up to 60% of term and 80% of preterm infants. It is often physiologic due to i...

Erythrocytes (red blood cells, RBCs) are anucleate, biconcave discoid cells that transport oxygen from the lungs to tissues and carry CO2 back to the lungs, primarily via hemoglobin. They have a lifespan of ~120 days and are produced in...

An aortic aneurysm is a permanent, localized dilation of the aorta to ≥1.5x its normal diameter, most commonly due to atherosclerosis (abdominal) or cystic medial degeneration/hypertension (thoracic). Abdominal aortic aneurysms (AAA) are...

Atrial flutter is a supraventricular tachyarrhythmia caused by a macro-reentrant circuit typically in the right atrium, most commonly circling the tricuspid valve annulus (cavotricuspid isthmus). It classically produces a 'sawtooth' patt...

Diastolic murmurs occur between S2 and S1 (during ventricular filling) and are almost always pathologic, indicating structural heart disease. The two classic causes are aortic/pulmonic regurgitation (early diastolic, decrescendo) and mit...

Anaplastic thyroid carcinoma is a rare, highly aggressive, undifferentiated thyroid malignancy typically occurring in elderly patients, often arising from dedifferentiation of pre-existing differentiated thyroid cancer (papillary or foll...

The rectum is the terminal ~12-15 cm portion of the large intestine, extending from the sigmoid colon (at S3) to the anal canal. It serves as a distensible reservoir for feces before defecation and has distinct arterial, venous, and lymp...

Pyruvate dehydrogenase complex (PDC) is a mitochondrial multi-enzyme complex that converts pyruvate to acetyl-CoA, linking glycolysis to the TCA cycle. It requires five cofactors: thiamine (B1/TPP), riboflavin (B2/FAD), niacin (B3/NAD+),...

Levothyroxine (T4) is a synthetic thyroid hormone used to treat hypothyroidism and TSH suppression in thyroid cancer. It is converted peripherally to the active form T3 and works by binding nuclear thyroid hormone receptors to regulate m...

Hepatocellular carcinoma (HCC) is the most common primary malignant liver tumor, strongly associated with chronic liver disease (cirrhosis from HBV, HCV, alcohol, NAFLD, hemochromatosis, aflatoxin exposure). It presents with symptoms of...

Ubiquitin is a small (76 amino acid) regulatory protein that tags other proteins for degradation via the ubiquitin-proteasome pathway. Covalent attachment of ubiquitin chains (polyubiquitination) marks damaged, misfolded, or short-lived...

Neoplasia refers to the abnormal, uncontrolled proliferation of cells that persists even in the absence of the stimuli that initiated it. This growth can be benign or malignant, resulting in a mass called a neoplasm (tumor). Neoplasia ar...

The ectoderm is the outermost of the three primary germ layers formed during gastrulation. It gives rise to the epidermis, nervous system (via neural tube and neural crest), and sensory organs. Neural crest and neural tube derivatives ar...

Pyrimidines are nitrogenous bases with a single six-membered ring, including cytosine, thymine, and uracil. They pair with purines (A-T/A-U, G-C) via hydrogen bonds in DNA/RNA. Their synthesis and salvage pathways are key targets for che...

Herpes simplex virus (HSV) is a double-stranded DNA virus in the Herpesviridae family, existing as two types: HSV-1 (classically oral/orolabial) and HSV-2 (classically genital), though both can cause either site of infection. The hallmar...

Heart failure with preserved ejection fraction (HFpEF) is a clinical syndrome of heart failure symptoms (dyspnea, edema, fatigue) with a normal or near-normal left ventricular ejection fraction (≥50%), caused by diastolic dysfunction—imp...

The bacterial cell wall is a rigid outer structure composed of peptidoglycan that maintains cell shape, provides osmotic protection, and is a key target for antibiotics. Its composition differs significantly between Gram-positive (thick...

An action potential (AP) is a rapid, transient depolarization-repolarization cycle in excitable cells (neurons, cardiac, and skeletal muscle) caused by sequential opening and closing of voltage-gated ion channels. It is an all-or-none ev...

The neuromuscular junction (NMJ) is the synapse between a motor neuron and skeletal muscle fiber, where acetylcholine (ACh) release triggers muscle contraction. It's the key target in diseases like myasthenia gravis and Lambert-Eaton syn...