Medical terms, defined.

Slipped capital femoral epiphysis (SCFE) is displacement of the femoral head (epiphysis) posteriorly and inferiorly relative to the femoral neck through the physis (growth plate). It classically affects obese adolescents (age 10-16, boys...

REM sleep behavior disorder (RBD) is a parasomnia characterized by loss of normal REM atonia, causing patients to physically act out vivid, often violent dreams (punching, kicking, yelling). It is strongly associated with alpha-synuclein...

Anti-epileptic drugs (AEDs, also called anticonvulsants or anti-seizure medications) suppress seizure activity by reducing neuronal excitability. Main mechanisms: blocking voltage-gated Na+ channels, enhancing GABA-mediated inhibition, b...

Cervical carcinoma is a malignancy of the uterine cervix, caused almost always by persistent high-risk HPV infection (types 16 and 18; E6 inactivates p53, E7 inactivates Rb). Squamous cell carcinoma is the most common type (~70-80%), ari...

NaCl is sodium chloride (table salt), the principal extracellular salt and main determinant of ECF osmolality and volume. Clinically it is given as IV fluid: 0.9% ("normal") saline is isotonic (154 mEq/L Na+ and Cl-, ~308 mOsm/L), and 3%...

Abortion is the termination of a pregnancy before fetal viability (<20 weeks gestation or <500 g fetal weight). It may be spontaneous (miscarriage; ~10–20% of clinically recognized pregnancies, most commonly from fetal chromosomal abnorm...

Eosinophils are bilobed, bright red (eosinophilic) granulocytes that defend against helminths and mediate type I hypersensitivity/allergic inflammation. They are recruited by IL-5 (also IL-3, GM-CSF, eotaxin) and contain major basic prot...

The midbrain (mesencephalon) is the smallest and most superior part of the brainstem, connecting the pons to the diencephalon. It contains the tectum (superior colliculi for visual reflexes, inferior colliculi for auditory), tegmentum (C...

Plasmodium spp. are obligate intracellular protozoan parasites that cause malaria, transmitted by the female Anopheles mosquito. Five species infect humans: P. falciparum (most severe, cerebral malaria, irregular fever), P. vivax and P....

A seizure is a transient episode of abnormal, excessive, hypersynchronous neuronal activity in the brain causing sudden changes in motor, sensory, autonomic, or cognitive function or consciousness. Seizures are classified as focal (one h...

Lipomas are the most common benign soft tissue tumors, composed of mature adipocytes. They present as soft, mobile, painless, slow-growing subcutaneous masses (often trunk, neck, proximal extremities) and are treated by observation or ex...

Potency is the amount of drug required to produce a given effect (usually 50% of maximal effect, i.e., EC50). A drug with a lower EC50 is more potent, shown as a leftward shift of the dose-response curve. Potency is distinct from efficac...

The cavernous sinus is a paired dural venous sinus on either side of the sella turcica/sphenoid body that drains the ophthalmic veins, superficial middle cerebral vein, and sphenoparietal sinus into the petrosal sinuses (then IJV). It co...

Secondary (central) hypothyroidism is thyroid hormone deficiency caused by inadequate pituitary TSH secretion, with a structurally normal thyroid gland. Labs show low free T4 with low or inappropriately normal TSH (contrast with primary...

Absence (petit mal) seizures are generalized non-motor seizures of childhood (ages ~4-12) with brief (<10-20 s) staring spells, impaired awareness, no postictal state, and 3-Hz spike-and-wave EEG. First-line treatment is ethosuximide (if...

Primary polydipsia is excessive fluid intake (>3 L/day) not driven by physiologic thirst, typically from psychiatric illness (e.g., schizophrenia) or hypothalamic dysfunction of the thirst center. It causes dilute urine, low serum osmola...

Complex partial seizure (current ILAE term: focal impaired awareness seizure) is a focal seizure with impaired consciousness/awareness, most often arising from the temporal lobe. It typically features an aura, automatisms (lip smacking,...

Benign paroxysmal positional vertigo (BPPV) is the most common cause of peripheral vertigo, caused by displaced otoconia (canaliths) in the semicircular canals, most often the posterior canal. It presents with brief (<1 min) episodes of...

D-transposition of the great arteries (d-TGA) is a cyanotic congenital heart defect in which the aorta arises from the right ventricle and the pulmonary artery from the left ventricle, creating two parallel (rather than in-series) circul...

Primary hyperthyroidism is excess thyroid hormone (T3/T4) production originating from the thyroid gland itself, giving low TSH with high free T4 and/or T3. Most common cause is Graves disease (TSI antibodies); others include toxic multin...

GI bleeding is hemorrhage anywhere along the GI tract. It is classified as upper (proximal to the ligament of Treitz; presents with hematemesis/melena) or lower (distal; presents with hematochezia). The most common causes are peptic ulce...

Bowel obstruction is a mechanical or functional blockage that impairs forward passage of intestinal contents. Small bowel obstruction (SBO) is most commonly caused by postoperative adhesions (then hernias and malignancy), whereas large b...

Status epilepticus (SE) is a neurologic emergency defined as a seizure lasting ≥5 minutes, or ≥2 seizures without return to baseline consciousness in between. Treat immediately: IV benzodiazepine (lorazepam) first-line, followed by a sec...

Contact dermatitis is skin inflammation caused by exposure to an external agent. It is either irritant (non-immune, most common, direct keratinocyte damage) or allergic (type IV delayed hypersensitivity, T-cell mediated, requires prior s...

A tonic-clonic seizure (formerly "grand mal") is a generalized seizure with loss of consciousness, a tonic phase of sustained muscle rigidity followed by a clonic phase of rhythmic jerking, then a postictal state of confusion and letharg...

Schizoaffective disorder is a chronic psychotic disorder in which a major mood episode (depressive or manic) occurs concurrently with schizophrenia-criterion-A symptoms, AND delusions or hallucinations persist for ≥2 weeks in the ABSENCE...

Toxic epidermal necrolysis (TEN) is a life-threatening, drug-induced mucocutaneous reaction (type IV hypersensitivity, CD8+ T-cell/granulysin-mediated keratinocyte apoptosis) causing full-thickness epidermal sloughing of >30% BSA with mu...

Antisocial personality disorder (ASPD) is a Cluster B personality disorder defined by a pervasive pattern of disregard for and violation of others' rights, beginning by age 15 (with evidence of conduct disorder before age 15), diagnosed...

Borderline personality disorder (BPD) is a Cluster B personality disorder characterized by a pervasive pattern of unstable relationships, self-image, and affect, with marked impulsivity, beginning by early adulthood. Classic features are...

Superior vena cava (SVC) syndrome is a set of signs and symptoms caused by obstruction of blood flow through the SVC, most commonly from malignancy (small cell lung cancer, non-Hodgkin lymphoma) or intravascular catheters/devices causing...

Somites are paired blocks of paraxial mesoderm that form along the neural tube starting in week 3 of embryonic development (about 42–44 pairs total). They differentiate into sclerotome (vertebrae and ribs), myotome (skeletal muscle), and...

Branchial (pharyngeal) pouches are 4 paired endoderm-lined outpocketings of the primitive pharynx that form between the pharyngeal arches. They give rise to the middle ear/auditory tube, palatine tonsils, thymus, and parathyroids. Rememb...

Pseudostratified epithelium is a single layer of cells that all contact the basement membrane but have nuclei at different heights, giving a falsely "stratified" appearance. The classic type is pseudostratified ciliated columnar epitheli...

Branchial (pharyngeal) clefts are ectoderm-lined grooves between the pharyngeal arches. Only the 1st cleft persists in normal development (external auditory meatus); clefts 2-4 are obliterated by the 2nd arch overgrowth (cervical sinus)....

Branchial (pharyngeal) arches are six paired mesenchymal structures (the 5th regresses, so arches 1, 2, 3, 4, 6 persist) that form in the 4th–5th week of embryogenesis and give rise to the bones, muscles, nerves, and arteries of the head...

Simple squamous epithelium is a single layer of flat, thin cells specialized for passive diffusion, filtration, and creating a slick, low-friction surface. It lines the alveoli, capillaries/blood vessels (endothelium), serous body caviti...

The umbilical arteries are paired fetal vessels (normally 2 per cord) that carry deoxygenated blood from the fetus to the placenta. They arise from the internal iliac arteries; after birth, the distal portions obliterate to form the medi...

Pharyngeal (branchial) clefts are ectoderm-lined grooves between the pharyngeal arches on the lateral surface of the embryo. Only the 1st cleft persists in adults, forming the external auditory meatus; clefts 2–4 are normally obliterated...

Transitional epithelium (urothelium) is a stratified epithelium lining the renal calyces, renal pelvis, ureters, bladder, and proximal urethra. Its surface "umbrella cells" and specialized plasma membrane allow distension and recoil whil...

Elastic cartilage is a flexible, resilient cartilage type containing abundant elastic fibers (elastin) plus type II collagen in its matrix. It is found in the external ear (auricle), external auditory canal, Eustachian tube, epiglottis,...

Loose (areolar) connective tissue is a pliable, sparsely packed connective tissue with abundant ground substance and a loose meshwork of collagen type I, elastic, and reticular fibers. It surrounds and cushions organs, vessels, and nerve...

Dense connective tissue is a connective tissue with abundant, tightly packed collagen fibers and relatively few cells (mainly fibroblasts/fibrocytes) and little ground substance. It provides tensile strength and is divided into dense reg...

Angiotensin receptor blockers (ARBs, "-sartans" e.g., losartan, valsartan) selectively block the AT1 receptor, preventing angiotensin II-mediated vasoconstriction, aldosterone release, and sympathetic activation. They treat hypertension,...

Fibrocartilage is a dense, tough connective tissue composed of type I (and type II) collagen with chondrocytes in lacunae, lacking a perichondrium. It resists compression and tensile/shear forces and is found in intervertebral discs (ann...

Hyaline cartilage is the most abundant cartilage type, composed of type II collagen and proteoglycans (aggrecan) in a glassy, translucent matrix. It lines articular surfaces of synovial joints and forms the fetal skeleton template (endoc...

Apoptotic bodies are membrane-bound vesicles containing condensed cytoplasm and nuclear fragments (karyorrhexis) that form when a cell undergoes programmed cell death (apoptosis). They display "eat-me" signals (e.g., phosphatidylserine)...

A Councilman body is an eosinophilic, round-to-oval, apoptotic hepatocyte seen in acute viral hepatitis (classically hepatitis B, also A, C, and yellow fever). It represents a shrunken, hypereosinophilic cell with fragmented or absent nu...

Auer rods are needle-shaped, azurophilic cytoplasmic inclusions in myeloblasts, composed of fused primary (azurophilic) granules containing myeloperoxidase and lysosomal enzymes. They are pathognomonic for acute myeloid leukemia (AML) an...

Call-Exner bodies are small, eosinophilic, fluid-filled spaces (containing basement membrane material) arranged between granulosa cells in a rosette pattern. They are the classic histologic hallmark of ovarian granulosa cell tumors, whic...

SSRIs (fluoxetine, sertraline, paroxetine, citalopram, escitalopram, fluvoxamine) block the presynaptic serotonin transporter (SERT), increasing synaptic 5-HT. They are first-line for major depression, anxiety disorders, OCD, PTSD, panic...