1500+ medical terms, defined.

Trisomy 18 (Edwards syndrome) is a chromosomal disorder caused by an extra copy of chromosome 18, resulting in severe developmental abnormalities and high mortality. It is the second most common autosomal trisomy after Down syndrome, wit...

Williams syndrome is a genetic disorder caused by a microdeletion on chromosome 7q11.23, which includes the elastin gene. It is characterized by intellectual disability, distinctive 'elfin' facial features, cardiovascular abnormalities (...

Stratified squamous epithelium consists of multiple layers of flattened cells, providing protection against abrasion and mechanical stress. It is found in areas requiring durability, either keratinized (skin/epidermis) or non-keratinized...

Goblet cells are specialized columnar epithelial cells found in the respiratory and GI tracts that secrete mucin to form protective mucus. They are a hallmark of mucus-secreting epithelium and undergo hyperplasia in various pathological...

Nondisjunction is the failure of homologous chromosomes (Meiosis I) or sister chromatids (Meiosis II or mitosis) to separate properly during cell division, resulting in gametes or cells with an abnormal chromosome number (aneuploidy). It...

A crescendo-decrescendo (diamond-shaped) murmur is a systolic ejection murmur that rises in intensity and then falls, classically heard in aortic stenosis. It is best heard at the right upper sternal border and radiates to the carotids.

The cardiac action potential is the sequence of ion channel-mediated voltage changes that trigger cardiac myocyte contraction. Ventricular/atrial myocytes have a 5-phase action potential (0-4) driven by Na+, Ca2+, and K+ currents, while...

The internal iliac artery is the main blood supply to the pelvic organs, gluteal region, and perineum. It arises from the bifurcation of the common iliac artery at the L5-S1 level and divides into anterior and posterior divisions. It's c...

Beta blockers are antagonists of beta-adrenergic receptors used to treat hypertension, angina, heart failure, arrhythmias, and post-MI to reduce mortality. They decrease heart rate, contractility, and myocardial oxygen demand, and can be...

Fatty acids are long hydrocarbon chains with a terminal carboxyl group that serve as key energy substrates, membrane components, and signaling molecules. They are classified as saturated (no double bonds) or unsaturated (one or more doub...

Decompensated heart failure is an acute worsening of heart failure signs and symptoms (e.g., dyspnea, edema, fatigue) requiring urgent medical intervention, often necessitating hospitalization. It represents a state where the heart can n...

A tracheoesophageal fistula (TEF) is an abnormal connection between the trachea and esophagus, most commonly occurring as a congenital anomaly associated with esophageal atresia. The most common variant is esophageal atresia with distal...

Retinoblastoma is the most common primary intraocular malignancy in children, typically presenting before age 5 with leukocoria (white pupillary reflex) or strabismus. It arises from biallelic loss of the RB1 tumor suppressor gene, follo...

Juxtaglomerular (JG) cells are modified smooth muscle cells located in the wall of the afferent arteriole of the renal glomerulus. They synthesize, store, and release renin in response to decreased renal perfusion pressure, decreased NaC...

Streptococcus bovis (now classified within the Streptococcus gallolyticus/Streptococcus equinus complex) is a group D, gram-positive, catalase-negative coccus found in the GI tract. It is classically associated with subacute bacterial en...

Viral pneumonia is an infection of the lung parenchyma caused by respiratory viruses (e.g., influenza, RSV, adenovirus, parainfluenza, SARS-CoV-2), leading to interstitial inflammation. It typically presents with a subacute onset of feve...

Bacterial pneumonia is an infection of the lung parenchyma caused by bacteria, leading to alveolar consolidation with inflammatory exudate. It presents with fever, productive cough, pleuritic chest pain, and dyspnea, and is diagnosed via...

Aquaporins are membrane channel proteins that facilitate rapid, selective transport of water across cell membranes in response to osmotic gradients. Aquaporin-2 (AQP2) in the renal collecting duct is the key target of ADH (vasopressin),...

Osmosis is the passive movement of water across a semipermeable membrane from an area of lower solute concentration to an area of higher solute concentration, driven by the difference in osmotic pressure. This process is fundamental to f...

Ascorbic acid (Vitamin C) is a water-soluble vitamin essential as a cofactor for prolyl and lysyl hydroxylase in collagen synthesis, and it acts as a potent antioxidant. Deficiency causes scurvy, characterized by poor wound healing, blee...

Glomerular filtration is the process by which the kidneys filter blood plasma across the glomerular capillary membrane into Bowman's space, forming an ultrafiltrate that becomes urine. It is driven by Starling forces (hydrostatic and onc...

The pectinate (dentate) line marks the transition from hindgut-derived columnar epithelium to ectoderm-derived squamous epithelium in the anal canal, roughly 2 cm inside the anus. It is a key anatomical landmark that determines differenc...

Glutathione peroxidase (GPx) is a selenium-containing antioxidant enzyme that reduces hydrogen peroxide and lipid peroxides to water and lipid alcohols, respectively, using glutathione (GSH) as the electron donor. It protects red blood c...

Methionine synthase is a vitamin B12 (cobalamin)-dependent enzyme that converts homocysteine to methionine using a methyl group donated by 5-methyltetrahydrofolate (5-methyl-THF). This reaction regenerates tetrahydrofolate (THF) for the...

Alanine aminotransferase (ALT) is a liver-specific enzyme that catalyzes the transfer of an amino group from alanine to α-ketoglutarate, producing pyruvate and glutamate. It is a key marker of hepatocellular injury, being more specific t...

Lymph nodes are small, encapsulated secondary lymphoid organs distributed along lymphatic vessels that filter lymph and serve as sites for immune surveillance and adaptive immune response initiation. They contain B-cell rich follicles (c...

Chromaffin cells are neuroendocrine cells located primarily in the adrenal medulla that synthesize, store, and secrete catecholamines (epinephrine and norepinephrine) in response to sympathetic stimulation. They are derived from neural c...

Trinucleotide repeat expansion disorders result from abnormal amplification of a 3-nucleotide sequence within a gene, leading to unstable, expandable repeats. These disorders often show anticipation (increasing severity/earlier onset in...

Chloride (Cl⁻) is the major extracellular anion, essential for maintaining plasma osmolality, acid-base balance, and electroneutrality alongside sodium. Serum reference range is approximately 96-106 mEq/L. Chloride disturbances often par...

The fallopian tubes (uterine tubes/oviducts) are paired muscular ducts that transport the oocyte from the ovary to the uterus and are the typical site of fertilization. They consist of four segments: infundibulum (with fimbriae), ampulla...

Pharyngeal (branchial) arches are paired embryonic structures that form during weeks 4-5 of development, giving rise to much of the head and neck. There are 6 arches (arch 5 is transient/absent), each with its own cartilage, muscle, nerv...

The vertebral arteries arise from the subclavian arteries, ascend through the transverse foramina of C6-C1, and join intracranially to form the basilar artery, supplying the posterior circulation of the brain including the brainstem, cer...

Bile salts are amphipathic molecules derived from cholesterol that emulsify dietary fats, aiding digestion and absorption of lipids and fat-soluble vitamins (A, D, E, K). They are synthesized in the liver, stored/concentrated in the gall...

Renal calculi (kidney stones) are solid mineral concretions that form in the kidney or urinary tract, most commonly composed of calcium oxalate. Patients classically present with acute, severe colicky flank pain radiating to the groin, h...

The lesser sac (omental bursa) is a peritoneal space located posterior to the stomach and lesser omentum, connecting to the greater sac via the epiploic foramen (Winslow's foramen). It allows the stomach to move freely against posterior...

Caput medusae refers to dilated, engorged periumbilical veins radiating outward from the umbilicus, resembling the snakes of Medusa's head. It is a sign of portal hypertension, resulting from recanalization of the paraumbilical veins and...

Lactate dehydrogenase (LDH) is a cytosolic enzyme found in nearly all body tissues that catalyzes the interconversion of pyruvate and lactate, using NADH/NAD+ as cofactors. It is a nonspecific marker of cellular injury or tissue breakdow...

Hantavirus is an RNA virus (family Bunyaviridae/Hantaviridae) transmitted to humans via inhalation of aerosolized rodent excreta (urine, feces, saliva), particularly from deer mice (Peromyscus maniculatus) in the Americas. It causes two...

Tubuloglomerular feedback (TGF) is an intrinsic renal autoregulatory mechanism that maintains constant GFR by adjusting afferent arteriolar tone based on the NaCl concentration sensed at the macula densa in the distal tubule. Increased d...

The dorsal root is the sensory (afferent) branch of a spinal nerve, carrying peripheral sensory input into the spinal cord. It joins the ventral (motor) root at the intervertebral foramen to form the mixed spinal nerve. The dorsal root g...

The morula is a solid ball of 12-16 blastomeres formed by cleavage of the zygote, appearing around day 3-4 post-fertilization before it enters the uterine cavity. It precedes formation of the blastocyst and is a key stage in early embryo...

Loss of heterozygosity (LOH) refers to the loss of one functional allele of a gene in a cell that was previously heterozygous, leaving only the mutated or non-functional allele. This is a key mechanism in tumor suppressor gene inactivati...

The pineal gland is a small, midline endocrine structure in the brain that secretes melatonin, regulating circadian rhythms in response to light/dark cycles. It is located in the epithalamus, posterior to the third ventricle. Pineal tumo...

Angelman syndrome is a neurodevelopmental disorder caused by loss of function of the maternally inherited UBE3A gene on chromosome 15q11-13. It presents with severe intellectual disability, ataxic movements, seizures, and inappropriate h...

'≥ 6 months' is a duration threshold commonly used in clinical diagnostic criteria to distinguish chronic conditions from acute or subacute presentations. It appears across many fields of medicine (psychiatry, nephrology, hepatology, rhe...

Carnitine is a small molecule required to shuttle long-chain fatty acids across the inner mitochondrial membrane for beta-oxidation, using the carnitine shuttle system (CPT-I, CPT-II, and translocase). Carnitine deficiency (primary genet...

Colesevelam is a bile acid sequestrant used primarily to lower LDL cholesterol and, secondarily, to improve glycemic control in type 2 diabetes. It works by binding bile acids in the intestine, preventing their reabsorption and promoting...

Transaminases (aminotransferases) are enzymes that catalyze the transfer of amino groups between amino acids and keto acids, playing a key role in amino acid metabolism and gluconeogenesis. The two clinically important ones are ALT (alan...

Elastase is a serine protease (from pancreatic acinar cells or neutrophils) that degrades elastin and other proteins. Pancreatic elastase (secreted as proelastase, activated by trypsin) aids protein digestion, while neutrophil elastase c...

Volume of distribution (Vd) is a pharmacokinetic parameter that relates the total amount of drug in the body to the concentration measured in plasma. It reflects the extent to which a drug distributes into tissues versus remaining in the...