300+ medical terms, defined.

Cancer is a group of diseases characterized by uncontrolled cellular growth and proliferation due to genetic mutations that disrupt normal cell cycle regulation. Malignant cells can invade surrounding tissues and metastasize to distant s...

Uniparental disomy (UPD) occurs when both copies of a chromosome (or chromosome segment) are inherited from the same parent instead of one from each parent. This can lead to genetic disorders through genomic imprinting effects or unmaski...

Placenta increta is an abnormally adherent placenta where chorionic villi invade into the myometrium but do not penetrate through to the serosa. It represents the intermediate form of placenta accreta spectrum disorders, occurring in abo...

Pleuritic chest pain is sharp, stabbing chest pain that worsens with inspiration, coughing, or movement due to inflammation of the pleural membranes. It is a key symptom that helps differentiate pulmonary causes from cardiac causes of ch...

Sphingomyelin is a phospholipid composed of sphingosine, fatty acid, and phosphocholine that is a major component of cell membranes, particularly in myelin sheaths and nervous tissue. Deficiency of sphingomyelinase enzyme leads to Nieman...

The notochord is a flexible, rod-like structure that serves as the primitive axial skeleton in early embryonic development. It induces neural tube formation and is eventually replaced by the vertebral column, with remnants persisting as...

The space of Disse is the perisinusoidal space between hepatocytes and liver sinusoidal endothelial cells, containing the extracellular matrix and serving as the site of lymph formation and metabolic exchange. It's clinically significant...

The falciform ligament is a sickle-shaped peritoneal fold that connects the liver to the anterior abdominal wall and diaphragm. It contains the ligamentum teres (round ligament of the liver), which is the obliterated umbilical vein, and...

The superior laryngeal nerve is a branch of the vagus nerve (CN X) that divides into internal and external branches. The internal branch provides sensory innervation to the supraglottis, while the external branch (motor) innervates the c...

Hepatic steatosis is the accumulation of triglycerides within hepatocytes, producing a 'fatty liver' grossly enlarged, yellow, and greasy. It is most commonly caused by alcohol use, obesity/metabolic syndrome (NAFLD), and is reversible i...

Tonofilaments are intermediate filaments composed of cytokeratin found in epithelial cells, especially keratinocytes. They insert into desmosomes and hemidesmosomes, providing mechanical strength to epithelia.

The most common malignant ovarian epithelial tumor, often bilateral, with characteristic psammoma bodies and a papillary architecture. Associated with BRCA1/2 mutations and p53 (TP53) mutations in high-grade lesions.

The most common benign primary CNS tumor in adults, arising from arachnoid cap cells of the meninges. Classic features: dural attachment, whorled pattern, psammoma bodies, women > men, NF2 association.

A rare neuroendocrine tumor (usually pancreatic delta-cell origin) that secretes somatostatin, producing the classic triad of diabetes, cholelithiasis, and steatorrhea. Somatostatin inhibits insulin, CCK, and pancreatic enzyme secretion.

The most common thyroid cancer (~80%), with excellent prognosis and lymphatic spread. Classic histology: 'Orphan Annie eye' nuclei, nuclear grooves, and psammoma bodies; risk factor is childhood ionizing radiation.

Bronchogenic carcinoma is lung cancer arising from bronchial epithelium and is the leading cause of cancer death in the US. Major subtypes: adenocarcinoma (most common overall and in nonsmokers), squamous cell carcinoma, small cell carci...

EML4-ALK is a fusion oncogene formed by inversion on chromosome 2 that drives a subset of non-small cell lung adenocarcinomas, particularly in younger nonsmokers. It is targetable by ALK tyrosine kinase inhibitors (crizotinib, alectinib).

An immune-mediated CNS syndrome triggered by an underlying tumor (classically small cell lung cancer) via anti-Hu (ANNA-1) antibodies. Causes subacute multifocal neurologic deficits, often preceding the cancer diagnosis.

NCAM (Neural Cell Adhesion Molecule, CD56) is an immunoglobulin-superfamily glycoprotein expressed on neurons, NK cells, and neuroendocrine tumors. It is a key IHC marker for neuroendocrine differentiation and NK-cell lineage.

Neuron-specific enolase (NSE) is the gamma-gamma dimer of the glycolytic enzyme enolase, expressed in neurons and neuroendocrine cells. It serves as a serum and IHC marker for neuroendocrine tumors, particularly small cell lung carcinoma...

Chromogranin A is an acidic glycoprotein stored in dense-core secretory granules of neuroendocrine cells and is the most specific serum and IHC marker for neuroendocrine tumors. Classic associations: carcinoid, pheochromocytoma, small ce...

Synaptophysin is an integral membrane glycoprotein of small presynaptic vesicles in neurons and small synaptic-like vesicles in neuroendocrine cells. It is one of the most sensitive IHC markers for neuroendocrine differentiation.

Polycyclic aromatic hydrocarbons (PAHs) are fused-ring organic carcinogens generated by incomplete combustion of organic material (tobacco smoke, charred meat, soot, fossil fuels). They are bioactivated by CYP450 to DNA-binding epoxides...

Bronchial (pulmonary) hamartoma is the most common benign lung tumor, composed of disorganized mature tissue (cartilage, fat, smooth muscle, epithelium) native to the lung. Classic finding: peripheral 'coin lesion' with 'popcorn' calcifi...

Miosis is constriction of the pupil (<2 mm). It results from parasympathetic activation (CN III, sphincter pupillae) or sympathetic loss, and is a component of Horner syndrome and opioid toxicity.

Anhidrosis is the inability to sweat, resulting from interruption of sympathetic cholinergic innervation to eccrine sweat glands. It is a hallmark of Horner syndrome (with ipsilateral facial anhidrosis) and is also seen in autonomic neur...

Capacitance is the ability of a blood vessel to store volume for a given change in pressure (C = dV/dP). Veins have ~20x higher capacitance than arteries and hold ~70% of total blood volume, serving as the body's volume reservoir.

Osteoporosis is a reduction in bone mass with normal bone mineralization, leading to fragility fractures (vertebral compression, hip, distal radius). Defined by DEXA T-score <= -2.5; most common in postmenopausal women due to estrogen de...

Laryngotracheobronchitis ('croup') is an acute viral infection of the upper airway in children 6 months to 3 years, most often caused by parainfluenza virus type 1. Classic features: barking 'seal-like' cough, inspiratory stridor, hoarse...

Bronchopneumonia is a patchy, multifocal infection of the bronchi/bronchioles extending into adjacent alveoli, typically bilateral and basal. Common pathogens: Staphylococcus aureus, Haemophilus influenzae, Klebsiella, and Pseudomonas; o...

Third stage of lobar pneumonia (days 4-6) in which the consolidated lung becomes gray-brown, dry, and firm as red cells lyse and neutrophils/fibrinopurulent exudate predominate. Lung still has a liver-like consistency, hence 'hepatization.'

Second stage of lobar pneumonia (days 3-4) in which the lung becomes red, firm, and airless, resembling liver. Alveoli are filled with RBCs, neutrophils, and fibrin.

Paraneoplastic hypercalcemia driven by tumor secretion of PTH-related peptide (PTHrP), classically with squamous cell carcinoma of the lung. Labs show high Ca, low phosphate, low PTH, high PTHrP.

Cell type that continuously divides throughout life from a stem-cell pool, allowing rapid regeneration after injury. Examples include skin epidermis, GI mucosa, and bone marrow.

General term for inflammation of lung parenchyma, usually non-infectious. Common causes include hypersensitivity (e.g., farmer's lung), drug toxicity (amiodarone, bleomycin, methotrexate), radiation, and aspiration of chemical irritants.

Mean pulmonary artery pressure >20 mmHg at rest with elevated pulmonary vascular resistance and normal wedge pressure (≤15). Leads to RV hypertrophy and cor pulmonale; classic histology shows plexiform lesions and medial hypertrophy.

Form of emphysema affecting the central/proximal acinus (respiratory bronchioles) with sparing of distal alveoli, predominantly in the upper lobes. Strongly associated with cigarette smoking.

Pulmonary alveolar macrophages that phagocytose inhaled particulates and pathogens. Hemosiderin-laden dust cells in sputum/BAL are 'heart failure cells,' seen in chronic pulmonary edema.

Black carbon pigment deposited in pulmonary macrophages and hilar lymph nodes from inhaled soot/coal dust. Generally benign in urban dwellers; pathologic in coal workers' pneumoconiosis.

Most common cystic fibrosis mutation: a three-nucleotide deletion causing loss of phenylalanine at position 508 of CFTR. Results in misfolded protein degraded in the ER, with no functional channel reaching the apical membrane.

Combination CFTR modulator (lumacaftor + ivacaftor) for cystic fibrosis patients homozygous for F508del. Lumacaftor is a corrector improving CFTR folding/trafficking; ivacaftor is a potentiator increasing channel open probability.

Fraction of inspired oxygen, expressed as a decimal (room air = 0.21). Used to titrate supplemental oxygen and to calculate P/F ratio (PaO2/FiO2), a key ARDS criterion.

Acid that only partially dissociates in solution (pKa typically 3-7). Pharmacologic relevance: weak acids are uncharged (lipid-soluble, absorbed) at low pH and ionized (trapped) at high pH; alkalinizing urine traps them and accelerates r...

Diffuse, patchy inflammation of the alveolar septa/interstitium rather than alveolar consolidation. Caused by 'atypical' organisms (Mycoplasma, Chlamydia, Legionella, viruses); presents with dry cough, low-grade fever, and CXR worse than...

Heavy or prolonged menstrual bleeding (>80 mL/cycle or >7 days). Common causes include fibroids, adenomyosis, endometrial polyps/hyperplasia, von Willebrand disease, and anovulation; classic complication is iron-deficiency anemia.

Lightweight metal causing chronic beryllium disease/berylliosis, a granulomatous lung disease mimicking sarcoidosis. Classic exposures: aerospace, nuclear, and electronics industries.

Fibroblast growth factor family of signaling proteins regulating angiogenesis, mesoderm formation, and limb development. FGF-FGFR mutations cause skeletal dysplasias (e.g., achondroplasia from FGFR3 gain-of-function).

Increased renal sodium excretion. Driven physiologically by atrial natriuretic peptide (ANP/BNP), pressure natriuresis, and inhibition of sodium reabsorption (e.g., loop, thiazide, K-sparing diuretics, SGLT2 inhibitors).

Phenomenon in which chronic aldosterone excess (e.g., Conn syndrome) initially expands volume but is then 'escaped' by pressure natriuresis and ANP, preventing edema. Hypokalemia and metabolic alkalosis persist because they are not escaped.

Alkylating-type chemotherapy used in MOPP regimen for Hodgkin lymphoma and in some brain tumors. Acts as a weak MAO inhibitor; key toxicities include disulfiram-like reaction with alcohol, myelosuppression, and secondary leukemia/inferti...