1500+ medical terms, defined.

The aorticopulmonary (AP) septum is the spiral partition formed by neural crest-derived truncal and conal ridges that divides the embryonic truncus arteriosus into the separate aorta and pulmonary trunk. Its spiral orientation accounts f...

Neural crest cells are a transient, multipotent population of cells arising from the border of the neural tube during embryogenesis. They migrate extensively throughout the embryo and differentiate into a remarkably diverse array of tiss...

The fourth ventricle is a diamond-shaped CSF-filled cavity located between the brainstem (pons and medulla) anteriorly and the cerebellum posteriorly. It connects the cerebral aqueduct (Sylvius) above to the central canal of the spinal c...

Mallory-Weiss tear refers to a longitudinal mucosal laceration at the gastroesophageal junction or gastric cardia, classically caused by severe vomiting or retching. It presents with hematemesis following forceful vomiting and is strongl...

Transferrin is a plasma glycoprotein synthesized by the liver that binds and transports iron in the blood. It reflects iron status and hepatic synthetic function, and is commonly assessed via transferrin saturation and total iron-binding...

Anastrozole is a third-generation nonsteroidal aromatase inhibitor used in the treatment of hormone receptor-positive breast cancer in postmenopausal women. It works by blocking peripheral conversion of androgens to estrogens, reducing e...

Hospital-acquired pneumonia (HAP) is pneumonia occurring ≥48 hours after hospital admission, not incubating at admission. It is commonly caused by multidrug-resistant organisms including Pseudomonas aeruginosa, MRSA, and Enterobacteriace...

Posterior urethral valves (PUV) are congenital obstructive membranous folds in the posterior urethra of male infants, representing the most common cause of severe lower urinary tract obstruction in newborn males. They lead to bladder out...

Selenium is an essential trace element and cofactor for antioxidant enzymes, most notably glutathione peroxidase and thioredoxin reductase. It is critical for thyroid hormone metabolism (via deiodinases) and protects cells from oxidative...

Osteoblasts are mesenchymal-derived, bone-forming cells that synthesize and secrete osteoid (unmineralized bone matrix consisting of type I collagen and other proteins), which subsequently mineralizes to form new bone. They express alkal...

Diabetic autonomic neuropathy (DAN) is a chronic complication of diabetes mellitus caused by damage to autonomic (sympathetic and parasympathetic) nerve fibers, resulting in dysfunction of cardiovascular, gastrointestinal, genitourinary,...

Tinea versicolor (pityriasis versicolor) is a superficial fungal skin infection caused by Malassezia species (M. furfur), yeasts that are part of normal skin flora. It presents as hypo- or hyperpigmented, scaly macules/patches typically...

A disulfiram-like reaction occurs when certain medications inhibit aldehyde dehydrogenase, causing acetaldehyde accumulation upon alcohol ingestion. This leads to flushing, headache, nausea, vomiting, tachycardia, and hypotension. It mim...

Aconitase is a Fe-S cluster-containing enzyme that catalyzes the isomerization of citrate to isocitrate via cis-aconitate in the citric acid cycle (TCA cycle). It functions in the mitochondrial matrix as the second enzyme step of the cyc...

Onychomycosis is a fungal infection of the nail (usually toenails), most commonly caused by dermatophytes, particularly Trichophyton rubrum. It presents with thickened, discolored (yellow-brown), brittle, and dystrophic nails. Diagnosis...

Coenzyme A (CoA) is a nucleotide-derived cofactor essential for the transfer of acyl groups in metabolism, most notably forming acetyl-CoA. It is synthesized from pantothenic acid (vitamin B5), cysteine, and ATP, and plays a central role...

A heart murmur is an abnormal 'whooshing' or 'blowing' sound heard during auscultation of the heart, caused by turbulent blood flow through cardiac chambers, valves, or great vessels. Murmurs can be innocent (benign, common in children)...

Tinea corporis, commonly known as ringworm, is a superficial dermatophyte infection of the body (glabrous skin) presenting as pruritic, annular, erythematous plaques with central clearing and an active scaly border. It is caused most com...

Tinea pedis (athlete's foot) is a dermatophyte infection of the feet, most commonly caused by Trichophyton rubrum. It presents with interdigital scaling, maceration, pruritus, and can involve the soles in a 'moccasin' distribution.

Pulmonic stenosis is a narrowing of the pulmonary valve or outflow tract that obstructs right ventricular outflow, causing right ventricular hypertrophy and a systolic ejection murmur best heard at the left upper sternal border. It is co...

Hiatal hernia occurs when part of the stomach protrudes through the esophageal hiatus of the diaphragm into the thoracic cavity. There are two main types: sliding (95%, most common) and paraesophageal (5%). Sliding hiatal hernias are str...

The upper esophageal sphincter (UES) is a physiologic sphincter formed primarily by the cricopharyngeus muscle, located at the junction of the pharynx and esophagus. It remains tonically contracted at rest to prevent air from entering th...

The hypothalamic-pituitary axis (HPA) is the central neuroendocrine system regulating hormone production via feedback loops between the hypothalamus, pituitary gland, and peripheral endocrine organs. The hypothalamus secretes releasing/i...

Sjögren syndrome is a chronic autoimmune disorder characterized by lymphocytic infiltration of exocrine glands, classically causing dry eyes (keratoconjunctivitis sicca) and dry mouth (xerostomia). It predominantly affects middle-aged wo...

The muscularis externa is the smooth muscle layer of the GI tract wall responsible for peristalsis and mixing of luminal contents. It typically consists of an inner circular and outer longitudinal layer, with the myenteric (Auerbach's) p...

Fractional excretion of sodium (FENa) is a calculated value used to differentiate prerenal azotemia from intrinsic acute tubular necrosis (ATN) in patients with acute kidney injury. FENa <1% suggests prerenal causes (intact tubular sodiu...

A paraesophageal hernia (PEH) is a type of hiatal hernia in which the gastroesophageal junction remains in its normal position but part of the stomach (fundus) herniates through the diaphragmatic hiatus alongside the esophagus. It accoun...

Apple-green birefringence refers to the characteristic yellow-green color amyloid deposits display under polarized light after staining with Congo red dye. This finding is the gold standard histologic confirmation of amyloidosis, indicat...

Paired sac-like glandular structures located posterior to the bladder and superior to the prostate that produce ~60-70% of seminal fluid volume, rich in fructose (energy for sperm), prostaglandins, and clotting proteins (semenogelin). Th...

Methylcobalamin is one of two active coenzyme forms of vitamin B12 (the other being adenosylcobalamin), essential for the methionine synthase reaction that converts homocysteine to methionine. It is critical for DNA synthesis, myelin for...

A transcription factor is a protein that binds specific DNA sequences (promoters, enhancers, silencers) to regulate the rate of gene transcription by RNA polymerase. Many transcription factors are key developmental regulators (e.g., HOX...

Donepezil is a centrally acting acetylcholinesterase (AChE) inhibitor used as first-line symptomatic treatment for mild to moderate Alzheimer's disease. It increases synaptic acetylcholine levels in the brain to improve cognition, though...

Ketone bodies (acetoacetate, β-hydroxybutyrate, and acetone) are water-soluble molecules produced by the liver from fatty acid breakdown during states of low glucose availability (fasting, starvation, diabetic ketoacidosis). They serve a...

Opioid withdrawal is a syndrome resulting from abrupt cessation or reduction of opioids in a physically dependent individual, or from opioid antagonist administration. It presents with flu-like symptoms, GI distress, and autonomic hypera...

Complex III (cytochrome bc1 complex) is the third protein complex of the electron transport chain, transferring electrons from ubiquinol (CoQH2) to cytochrome c while pumping protons into the intermembrane space via the Q cycle. It is ta...

Cytochrome c oxidase (Complex IV) is the terminal enzyme of the mitochondrial electron transport chain, catalyzing the transfer of electrons from cytochrome c to molecular oxygen, reducing it to water. It is the direct target of cyanide...

Monoclonal antibodies (mAbs) are laboratory-engineered antibodies derived from a single clone of B cells, designed to bind a specific antigen with high specificity. They are widely used therapeutically (e.g., in oncology, autoimmune dise...

Dermatitis herpetiformis (DH) is an intensely pruritic, blistering skin disorder strongly associated with celiac disease, caused by IgA autoantibodies against tissue transglutaminase (and epidermal transglutaminase) that deposit at the d...

Cholesteryl ester transfer protein (CETP) is a plasma protein that facilitates the transfer of cholesteryl esters from HDL to LDL and VLDL in exchange for triglycerides. It plays a key role in reverse cholesterol transport and lipid meta...

Adjuvant therapy is treatment given after the primary treatment (usually surgery) to eliminate residual microscopic disease and reduce the risk of recurrence. Common modalities include chemotherapy, radiation, hormone therapy, and immuno...

A spermatocele is a benign, sperm-filled cyst that develops in the epididymis, typically at the head. It presents as a painless, smooth, cystic mass palpable separately from the testicle, and transilluminates on exam.

Upper motor neurons (UMNs) are neurons whose cell bodies reside in the motor cortex or brainstem and project via the corticospinal and corticobulbar tracts to synapse on lower motor neurons (LMNs) or interneurons in the spinal cord/brain...

Primary tuberculosis is the initial infection with Mycobacterium tuberculosis in a previously unexposed (immunologically naive) host, typically involving the mid/lower lung zones and hilar lymph nodes. It is usually asymptomatic or mild...

Zone 2 of the neck is the anatomic region extending from the cricoid cartilage to the angle of the mandible. It is the most commonly injured zone in penetrating neck trauma and is unique because it is surgically accessible, allowing dire...

Aneuploidy is a chromosomal abnormality characterized by an abnormal number of chromosomes that is not an exact multiple of the haploid set, typically resulting from nondisjunction during meiosis. Common examples tested on boards include...

Chemoreceptors are specialized sensory receptors that detect changes in the chemical composition of blood or interstitial fluid, particularly O2, CO2, and pH levels. They are classified as peripheral (carotid and aortic bodies) or centra...

HMG-CoA synthase catalyzes the condensation of acetyl-CoA and acetoacetyl-CoA to form HMG-CoA (3-hydroxy-3-methylglutaryl-CoA), a key intermediate at the branch point between cholesterol synthesis (cytosolic isoform) and ketone body prod...

Chylothorax is the accumulation of lymphatic fluid (chyle) in the pleural space due to disruption or obstruction of the thoracic duct. It presents as a milky-white pleural effusion rich in triglycerides (>110 mg/dL). Common causes includ...

Sustained isometric handgrip exercise increases myocardial oxygen demand by raising systemic vascular resistance, heart rate, and blood pressure via sympathetic activation. It is used clinically as a bedside maneuver to alter heart murmu...

Transesophageal echocardiography (TEE) is a semi-invasive ultrasound imaging technique using a probe advanced into the esophagus to obtain high-resolution images of the heart, particularly posterior structures like the left atrium, mitra...