Medical terms, defined.

The Wolffian (mesonephric) duct is the embryonic paired duct that, in males under testosterone influence, differentiates into the epididymis, vas deferens, seminal vesicles, ejaculatory duct (and efferent ductules). In females, lacking t...

Pleiotropy is when a single gene (or mutation) affects multiple, seemingly unrelated phenotypic traits or organ systems. It is a classic explanation for why one genetic defect produces a syndrome with diverse manifestations (e.g., PKU, M...

Epistasis is a gene interaction in which the phenotypic effect of one gene (the epistatic gene) masks or modifies the expression of a different gene at another locus (the hypostatic gene). It is distinct from dominance, which involves al...

VACTERL association is a nonrandom co-occurrence of congenital anomalies: Vertebral defects, Anal atresia, Cardiac defects, Tracheo-Esophageal fistula, Renal anomalies, and Limb (radial/preaxial) defects. It is a diagnosis of exclusion,...

The Müllerian (paramesonephric) ducts are embryonic paired ducts that, in the absence of AMH (anti-Müllerian hormone) and SRY/testosterone effects, develop into the fallopian tubes, uterus, cervix, and upper one-third of the vagina. In m...

Homoplasmy is the state in which all copies of mitochondrial DNA (mtDNA) within a cell or tissue are identical, either all wild-type or all carrying the same mutation. It contrasts with heteroplasmy, where normal and mutant mtDNA coexist...

Chimerism is the presence of two or more genetically distinct cell lines (derived from different zygotes) in a single individual. It arises naturally (twin-twin placental anastomoses, fusion of two zygotes, maternal-fetal cell exchange)...

Heteroplasmy is the coexistence of both normal (wild-type) and mutated mitochondrial DNA (mtDNA) within a single cell or individual. The proportion of mutant mtDNA determines whether disease manifests (threshold effect) and how severe it...

Cri du chat syndrome (5p- syndrome) is caused by a partial deletion of the short arm of chromosome 5 (5p15.2–15.3). It presents with a high-pitched cat-like cry in infancy, microcephaly, intellectual disability, hypotonia, and characteri...

Wolf-Hirschhorn syndrome is a contiguous gene deletion syndrome caused by a partial deletion of the short arm of chromosome 4 (4p16.3). It presents with the classic "Greek warrior helmet" facies, growth restriction, intellectual disabili...

The Knudson two-hit hypothesis states that both alleles of a tumor suppressor gene must be inactivated for tumor development. In hereditary forms, the first hit is a germline mutation (present in all cells) and the second hit is a somati...

Brief psychotic disorder is a psychotic illness with at least one of delusions, hallucinations, disorganized speech, or grossly disorganized/catatonic behavior lasting ≥1 day but <1 month, followed by full return to baseline functioning....

Depersonalization/derealization disorder (DPDR) is a dissociative disorder characterized by persistent or recurrent episodes of feeling detached from one's own self (depersonalization) and/or surroundings (derealization), with intact rea...

Schizophreniform disorder is a psychotic disorder with schizophrenia-like symptoms (delusions, hallucinations, disorganized speech/behavior, negative symptoms) lasting ≥1 month but <6 months. If symptoms persist ≥6 months, the diagnosis...

The odds ratio (OR) compares the odds of exposure among cases with the odds of exposure among controls (case-control study), or equivalently the odds of disease in exposed vs. unexposed. OR = ad/bc from a 2x2 table. It approximates relat...

Factitious disorder is the intentional production or feigning of physical or psychological symptoms to assume the "sick role," driven by an internal psychological need and not by external rewards (as in malingering). Two types: imposed o...

Conversion disorder (Functional Neurological Symptom Disorder in DSM-5) is characterized by one or more neurologic symptoms (motor or sensory deficits, seizures-like episodes) incompatible with recognized neurologic disease, causing dist...

Dissociative identity disorder (DID) is a dissociative disorder characterized by two or more distinct personality states (identities) that recurrently take control of behavior, with gaps in recall of everyday events, important personal i...

Number needed to treat (NNT) is the number of patients who must receive an intervention for one additional patient to benefit (or avoid a bad outcome) compared with control. NNT = 1 / absolute risk reduction (ARR). A lower NNT means a mo...

Number needed to harm (NNH) is the number of patients who must be exposed to a treatment or risk factor for one additional patient to experience an adverse outcome. It is calculated as 1/absolute risk increase (ARI). A lower NNH means th...

Munchausen by proxy, now termed Factitious Disorder Imposed on Another (DSM-5), is a form of child abuse (or abuse of a dependent) in which a caregiver, most often the mother, falsifies, exaggerates, or induces illness in the victim to g...

A likelihood ratio (LR) compares the probability of a given test result in patients with disease vs. without disease. LR+ = sensitivity / (1 − specificity); LR− = (1 − sensitivity) / specificity. Unlike predictive values, LRs do not depe...

Positive predictive value (PPV) is the probability that a person with a positive test result truly has the disease: PPV = TP / (TP + FP). Unlike sensitivity and specificity, PPV depends on disease prevalence: it increases when prevalence...

Lead-time bias is a screening-related bias in which early detection of disease makes survival appear longer, even though the time of death is unchanged. The "lead time" is the interval between early detection by screening and when the di...

Length-time bias is a type of screening bias in which slow-growing, less aggressive (indolent) disease is preferentially detected because it has a longer asymptomatic preclinical phase, so screening overestimates survival/benefit. Aggres...

Recall bias is a type of information (measurement) bias in which study participants with a disease or outcome remember or report past exposures differently than controls. It is most characteristic of retrospective case-control studies an...

Delusional disorder is a psychotic disorder characterized by one or more non-bizarre delusions lasting ≥1 month, without prominent hallucinations, disorganized speech, or marked functional impairment outside the delusion's direct effects...

Observer bias is a type of measurement (information) bias in which the investigator's or assessor's expectations, beliefs, or knowledge of a participant's exposure or treatment status systematically influence how outcomes are recorded or...

A case-control study is an observational, retrospective study that starts with outcome status: it compares people with the disease (cases) to people without it (controls) and looks back to compare prior exposures. It is best for rare dis...

Intention-to-treat (ITT) analysis analyzes all randomized participants in the groups to which they were originally assigned, regardless of adherence, crossover, or dropout. It preserves randomization, minimizes selection/attrition bias,...

A confounding variable is a third factor associated with both the exposure and the outcome, and it is not on the causal pathway between them. It distorts the apparent exposure–disease relationship, producing a spurious association or mas...

A crossover study is an experimental (usually randomized) design in which each participant receives every intervention in sequence, serving as their own control. A washout period between treatments prevents carryover effects. It reduces...

A cohort study is an observational study that follows groups (cohorts) defined by exposure status over time to compare the incidence of an outcome. It calculates relative risk (RR) and incidence, and can be prospective or retrospective....

A meta-analysis is a statistical technique that quantitatively combines results from multiple independent studies addressing the same question to produce a single pooled effect estimate. It sits at the top of the evidence hierarchy (with...

Zero-order kinetics describes elimination of a drug at a constant AMOUNT per unit time, regardless of plasma concentration, because the elimination mechanisms (enzymes/transporters) are saturated. Plasma concentration falls linearly with...

First-order kinetics describes drug elimination in which a constant FRACTION (percentage) of drug is eliminated per unit time, so the rate of elimination is directly proportional to plasma drug concentration. It applies to most drugs at...

A systematic review is a rigorous, reproducible synthesis of all relevant studies on a focused clinical question, using a predefined protocol with explicit search strategy, inclusion/exclusion criteria, and critical appraisal to minimize...

A randomized controlled trial (RCT) is an experimental study in which participants are randomly assigned to an intervention group or a control group (placebo or standard care) and followed prospectively for outcomes. Randomization minimi...

Maintenance dose is the dose of a drug given repeatedly to maintain a target steady-state plasma concentration by replacing the amount eliminated between doses. It depends on clearance (CL) and bioavailability (F): Maintenance dose = (CL...

A loading dose is a larger initial dose of a drug given to rapidly achieve target plasma concentration (steady state-like levels), rather than waiting ~4-5 half-lives. It is determined by volume of distribution (Vd) and desired target co...

First-pass metabolism (presystemic metabolism) is the extensive hepatic (and gut wall) biotransformation of an orally absorbed drug before it reaches systemic circulation, which reduces oral bioavailability (F). Drugs with high first-pas...

Phase I metabolism consists of functionalization reactions (oxidation, reduction, hydrolysis) that introduce or unmask a polar group (-OH, -NH2, -SH, -COOH) on a drug, mainly via the cytochrome P450 enzymes in the liver. Products are usu...

Phase II metabolism (conjugation) attaches polar endogenous groups (glucuronate, sulfate, glutathione, acetate, glycine, methyl) to drugs or Phase I metabolites, making them more water-soluble and generally inactive for renal or biliary...

First-pass effect (presystemic metabolism) is the metabolism of an orally administered drug by the gut wall and liver before it reaches systemic circulation, reducing oral bioavailability (F). Drugs with high hepatic extraction (e.g., ni...

Red man syndrome (vancomycin infusion reaction, now often called vancomycin flushing syndrome) is a rate-related, non-IgE-mediated pseudoallergic reaction caused by direct mast cell degranulation and histamine release. It presents with p...

The omentum is a double-layered peritoneal fold draping from the stomach and duodenum to other abdominal structures. The greater omentum (gastrocolic, gastrosplenic, gastrophrenic ligaments) hangs from the greater curvature of the stomac...

The femoral vein is the main deep vein of the thigh, continuing from the popliteal vein at the adductor hiatus and becoming the external iliac vein at the inguinal ligament. It lies medial to the femoral artery in the femoral triangle (N...

The jugular veins are paired large neck veins that drain blood from the head and neck to the brachiocephalic/subclavian veins and then the SVC. The internal jugular (IJ) is the main drainage of the brain and face, and the external jugula...

The sciatic nerve is the largest nerve in the body, formed from the L4–S3 ventral rami of the lumbosacral plexus. It exits the pelvis via the greater sciatic foramen inferior to the piriformis, supplies the posterior thigh (hamstrings) a...

The hippocampus is a curved archicortical (three-layered) structure in the medial temporal lobe that is essential for forming new declarative (explicit) memories and for spatial navigation. It is part of the limbic system and is highly v...