Medical terms, defined.

Homogentisic acid oxidase (homogentisate 1,2-dioxygenase, HGD) is the enzyme in the tyrosine degradation pathway that converts homogentisic acid to maleylacetoacetic acid. Its deficiency (autosomal recessive) causes alkaptonuria: dark ur...

Fructose-1,6-bisphosphatase (FBPase-1) is a rate-limiting enzyme of gluconeogenesis that converts fructose-1,6-bisphosphate to fructose-6-phosphate (releasing Pi). It is activated by ATP and citrate and inhibited by AMP and fructose-2,6-...

The Auerbach (myenteric) plexus is the enteric nervous system plexus located between the inner circular and outer longitudinal layers of the muscularis externa. It primarily controls GI motility (peristalsis), whereas the Meissner (submu...

Ampicillin-sulbactam (Unasyn) is an IV aminopenicillin plus a irreversible beta-lactamase inhibitor, giving broad coverage of gram-positive, gram-negative, and anaerobic organisms. Classic uses: polymicrobial skin/soft tissue, intra-abdo...

The myenteric (Auerbach) plexus is the enteric nervous system network located between the inner circular and outer longitudinal muscle layers of the muscularis externa. It primarily controls GI motility (peristalsis). Absence of its gang...

Levodopa-carbidopa is the first-line, most effective symptomatic therapy for Parkinson disease. Levodopa (L-DOPA) is the dopamine precursor that crosses the blood-brain barrier. Carbidopa is a peripheral DOPA decarboxylase inhibitor that...

The Meissner (submucosal) plexus is a component of the enteric nervous system located in the submucosa of the GI tract (small and large intestine). It primarily regulates local secretion, absorption, and blood flow, and it is absent in t...

Trimethoprim-sulfamethoxazole (TMP-SMX, co-trimoxazole, Bactrim) is a synergistic combination that sequentially blocks bacterial folate synthesis (sulfamethoxazole inhibits dihydropteroate synthase; trimethoprim inhibits dihydrofolate re...

Branched-chain alpha-ketoacid dehydrogenase (BCKDH) is a mitochondrial enzyme complex that oxidatively decarboxylates the alpha-ketoacids of leucine, isoleucine, and valine. Deficiency causes maple syrup urine disease (MSUD), an autosoma...

Southern blot is a laboratory technique that detects a specific DNA sequence in a sample. DNA is cut with restriction endonucleases, separated by gel electrophoresis, transferred to a membrane, and hybridized with a labeled complementary...

Northern blot is a molecular biology technique that detects specific RNA (mRNA) sequences in a sample, using a labeled DNA or RNA probe. It assesses gene expression (size and abundance of a transcript). Mnemonic: SNoW DRoP – Southern = D...

The AV node is a specialized cardiac conduction structure in the subendocardium of the interatrial septum near the coronary sinus ostium (Koch's triangle). It delays impulse conduction (~0.1 s; PR interval) so ventricles can fill before...

Gram-negative bacteria have a thin peptidoglycan layer sandwiched between an inner (cytoplasmic) membrane and an outer membrane containing lipopolysaccharide (LPS/endotoxin). They stain pink/red on Gram stain because they don't retain cr...

Gram-positive bacteria retain the crystal violet–iodine complex during Gram staining and appear purple/blue because they have a thick peptidoglycan layer and no outer membrane. They contain teichoic and lipoteichoic acids, and major path...

Prussian blue (Perls) stain is a histochemical stain that detects ferric iron (Fe3+, i.e., hemosiderin) in tissue, turning it blue. It is used to identify iron overload (hemochromatosis, hemosiderosis), ring sideroblasts in bone marrow,...

PCR (polymerase chain reaction) is an in vitro technique that exponentially amplifies a specific DNA sequence using primers, heat-stable DNA polymerase (Taq), dNTPs, and repeated thermal cycles (denaturation, annealing, extension). It is...

Generalized anxiety disorder (GAD) is a chronic condition marked by excessive, difficult-to-control worry about multiple everyday domains (work, health, family, finances) on most days for at least 6 months, with ≥3 somatic/cognitive symp...

Obsessive-compulsive disorder (OCD) is characterized by recurrent, intrusive, ego-dystonic obsessions (thoughts, urges, images) and/or compulsions (repetitive behaviors or mental acts performed to reduce anxiety). It is time-consuming (>...

REM sleep behavior disorder (RBD) is a parasomnia characterized by loss of normal skeletal muscle atonia during REM sleep, causing patients to physically act out vivid, often violent dreams. It is a strong prodromal marker of alpha-synuc...

Bipolar disorders are mood disorders defined by episodes of mania or hypomania, usually alternating with major depressive episodes. Bipolar I requires ≥1 manic episode (≥7 days or any duration if hospitalization is needed); Bipolar II re...

ADHD is a neurodevelopmental disorder marked by a persistent pattern of inattention and/or hyperactivity-impulsivity, with symptoms present before age 12, lasting ≥6 months, and causing impairment in ≥2 settings (e.g., home and school)....

Social anxiety disorder (SAD, social phobia) is marked, persistent fear of social or performance situations in which one may be scrutinized or judged negatively, lasting ≥6 months and causing impairment. First-line treatment is SSRIs/SNR...

Somatic symptom and related disorders (DSM-5) are psychiatric conditions in which physical symptoms cause significant distress or functional impairment, accompanied by excessive thoughts, feelings, or behaviors about the symptoms. The sy...

Oppositional defiant disorder (ODD) is a childhood-onset disruptive behavior disorder marked by a persistent pattern (≥6 months) of angry/irritable mood, argumentative/defiant behavior, and vindictiveness toward authority figures, withou...

Disruptive mood dysregulation disorder (DMDD) is a pediatric DSM-5 disorder characterized by severe, recurrent temper outbursts (verbal or behavioral, grossly out of proportion) occurring ≥3 times/week, with persistently irritable or ang...

PTSD is a trauma- and stressor-related disorder in which symptoms persist >1 month after exposure to actual or threatened death, serious injury, or sexual violence. It features intrusion (flashbacks, nightmares), avoidance, negative alte...

Major depressive disorder (MDD) is a mood disorder defined by ≥2 weeks of depressed mood and/or anhedonia plus other symptoms (SIG E CAPS: Sleep, Interest, Guilt, Energy, Concentration, Appetite, Psychomotor, Suicidality), with at least...

Alcohol use disorder (AUD) is a DSM-5 diagnosis defined by a problematic pattern of alcohol use causing clinically significant impairment or distress, with ≥2 of 11 criteria in 12 months (mild 2-3, moderate 4-5, severe ≥6). Chronic use c...

Persistent depressive disorder (PDD, formerly dysthymia) is a chronic depressed mood lasting ≥2 years in adults (≥1 year in children/adolescents, who may have irritable mood), with ≥2 additional depressive symptoms, and no symptom-free i...

Adjustment disorder is a maladaptive emotional or behavioral response to an identifiable stressor, beginning within 3 months of the stressor onset. It causes marked distress or functional impairment, does not meet criteria for another ps...

Panic disorder is an anxiety disorder characterized by recurrent, unexpected panic attacks (abrupt surge of intense fear peaking within minutes with somatic symptoms), followed by ≥1 month of persistent worry about further attacks or mal...

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

Schizoaffective disorder is a psychotic illness 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 of a maj...

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, chronic emptiness, and recurrent self-harm or suic...

Short QT syndrome (SQTS) is a rare, inherited channelopathy caused mainly by gain-of-function mutations in potassium channel genes. It produces a persistently shortened QTc (typically ≤340-360 ms) and predisposes to atrial fibrillation,...

Leriche syndrome is chronic aortoiliac occlusive disease (atherosclerotic occlusion of the distal abdominal aorta/common iliac arteries). Classic triad: bilateral buttock/thigh claudication, diminished or absent femoral pulses, and erect...

Libman-Sacks endocarditis is a sterile, nonbacterial verrucous endocarditis classically associated with systemic lupus erythematosus (SLE) and antiphospholipid syndrome. Small, warty vegetations form on both surfaces of the valves (most...

Arrhythmogenic right ventricular cardiomyopathy (ARVC) is an inherited (usually autosomal dominant) desmosomal disease in which RV (and sometimes LV) myocardium is replaced by fibrofatty tissue. It presents with ventricular arrhythmias (...

Hamman-Rich syndrome is an older eponym for acute interstitial pneumonia (AIP): a fulminant, idiopathic form of diffuse alveolar damage presenting as rapidly progressive dyspnea and ARDS-like respiratory failure. It has no identifiable c...

Light criteria distinguish exudative from transudative pleural effusions. An effusion is exudative if it meets ≥1 of: pleural fluid protein/serum protein >0.5, pleural fluid LDH/serum LDH >0.6, or pleural fluid LDH >2/3 the upper limit o...

A transudative effusion is a fluid collection (pleural, pericardial, peritoneal) caused by altered hydrostatic or oncotic pressure with intact capillary permeability, yielding low-protein, low-LDH fluid. Common causes are heart failure,...

An exudative effusion is a pleural (or other serous cavity) fluid collection caused by increased capillary permeability or impaired lymphatic drainage, producing protein- and LDH-rich fluid that meets Light's criteria. Common causes are...

Thin basement membrane disease (benign familial hematuria) is an autosomal dominant disorder caused by heterozygous mutations in COL4A3/COL4A4, leading to diffuse thinning of the glomerular basement membrane (GBM). It presents with persi...

Microscopic colitis is a cause of chronic, watery, non-bloody secretory diarrhea, typically in middle-aged to older women, with a NORMAL-appearing colonoscopy and diagnosis made by biopsy. Two subtypes: collagenous colitis (thickened sub...

Pulmonary alveolar proteinosis (PAP) is a rare diffuse lung disease in which surfactant-derived lipoproteinaceous material accumulates in the alveoli because alveolar macrophages can't clear it. Most cases (~90%) are autoimmune, caused b...

Type 1 (distal) RTA is a defect in H+ secretion by alpha-intercalated cells of the distal tubule/collecting duct, causing a normal anion gap (hyperchloremic) metabolic acidosis with inappropriately high urine pH (>5.5), hypokalemia, and...

Hepatopulmonary syndrome (HPS) is a triad of chronic liver disease (usually cirrhosis with portal hypertension), an increased alveolar-arterial O2 gradient (A-a gradient ≥15 mmHg, or ≥20 if age >64), and intrapulmonary vascular dilatatio...

Diabetic glomerulosclerosis (diabetic nephropathy) is glomerular damage from chronic hyperglycemia, and it's the leading cause of ESRD in the US. The classic lesion is nodular glomerulosclerosis (Kimmelstiel-Wilson nodules) with diffuse...

Schmidt syndrome is Autoimmune Polyglandular Syndrome type 2 (APS-2): autoimmune Addison disease (primary adrenal insufficiency) plus autoimmune thyroid disease (Hashimoto or Graves) and/or type 1 diabetes mellitus. It is polygenic, asso...

Sipple syndrome is another name for MEN type 2A: medullary thyroid carcinoma, pheochromocytoma, and primary hyperparathyroidism (parathyroid hyperplasia/adenoma). It is an autosomal dominant condition caused by gain-of-function germline...