1500+ medical terms, defined.

The direct Coombs test (direct antiglobulin test, DAT) detects antibodies or complement proteins bound directly to the surface of a patient's red blood cells in vivo. A positive result indicates immune-mediated hemolysis, as seen in auto...

Monocytes are the largest circulating leukocytes, part of the innate immune system, that differentiate into macrophages and dendritic cells upon migrating into tissues. They function in phagocytosis, antigen presentation, and cytokine pr...

Sézary syndrome is a leukemic, aggressive variant of cutaneous T-cell lymphoma characterized by the triad of erythroderma, generalized lymphadenopathy, and circulating atypical malignant T cells (Sézary cells) with cerebriform nuclei. It...

Cold agglutinin disease (CAD) is a type of autoimmune hemolytic anemia caused by IgM autoantibodies that bind red blood cells at cold temperatures, causing complement-mediated hemolysis and RBC agglutination. It's classically associated...

Auer rods are needle-shaped, eosinophilic cytoplasmic inclusions composed of fused primary (azurophilic) granules found in myeloblasts. They are a classic diagnostic clue for acute myeloid leukemia (AML), particularly the M3 subtype (acu...

The primitive atrium is the embryonic precursor structure that develops into the adult right and left atria during heart development. It arises from the cranial portion of the primitive heart tube around week 3-4 of embryogenesis and und...

B cells are lymphocytes derived from bone marrow that mediate humoral immunity by differentiating into plasma cells (which secrete antibodies) and memory B cells. They recognize antigens via surface immunoglobulin (BCR) and require antig...

Basophils are the least common granulocyte in peripheral blood (<1-2% of WBCs), containing large basophilic granules with histamine, heparin, and other mediators. They mediate allergic and hypersensitivity reactions, functionally similar...

Type I hypersensitivity is an immediate (within minutes) allergic reaction mediated by IgE antibodies binding to mast cells and basophils, causing degranulation upon re-exposure to an antigen (allergen). It underlies conditions like anap...

Amniotic fluid embolism (AFE) is a rare, catastrophic obstetric emergency in which amniotic fluid, fetal cells, hair, or debris enter the maternal circulation, triggering an anaphylactoid-like reaction. It classically presents during lab...

A sarcomere is the basic contractile unit of skeletal and cardiac muscle, extending from one Z-line to the next. It contains organized thick (myosin) and thin (actin) filaments that slide past each other to produce muscle contraction, pe...

A pilus (or fimbria) is a hair-like proteinaceous appendage on the surface of bacteria that mediates adherence to host cells and surfaces. Common (fimbrial) pili are key virulence factors for colonization, while sex pili facilitate conju...

Reticulocytes are immature red blood cells that still contain residual ribosomal RNA, reflecting recent release from the bone marrow. The reticulocyte count (especially the corrected reticulocyte count/reticulocyte production index) is u...

Pulmonary stenosis is narrowing of the pulmonary valve (or less commonly infundibular/supravalvular regions) causing obstruction to right ventricular outflow. It is a common congenital heart defect, often isolated or part of syndromes li...

Myocardial oxygen demand (MVO2) refers to the amount of oxygen required by heart muscle to sustain contractile function. It is determined by three primary factors: heart rate, contractility, and wall tension (afterload and preload). Unde...

Antimetabolites are a class of antineoplastic (and some immunosuppressive) drugs that structurally resemble normal metabolites (folate, purines, pyrimidines) and interfere with DNA/RNA synthesis, causing cell death primarily in the S pha...

Heart failure with reduced ejection fraction (HFrEF), also called systolic heart failure, is defined by an LVEF ≤40% due to impaired myocardial contractility. It results in decreased cardiac output and activation of neurohormonal compens...

Atypical (second-generation) antipsychotics are drugs used to treat schizophrenia, bipolar disorder, and other psychiatric conditions by antagonizing serotonin 5-HT2A receptors along with dopamine D2 receptors. Compared to typical antips...

Prone refers to a body position in which a person lies face down, with the front of the body oriented toward the supporting surface. It is the opposite of the supine position, in which a person lies face up. This positioning term is used...

Complex II (succinate dehydrogenase) is the only electron transport chain complex directly involved in the citric acid cycle, oxidizing succinate to fumarate while reducing FAD to FADH2 and transferring electrons to ubiquinone (CoQ). Unl...

Respiratory bronchioles are the transitional airways connecting terminal bronchioles to alveolar ducts, representing the first site of gas exchange in the respiratory tree. They mark the beginning of the respiratory zone of the lung, dis...

Subcutaneous tissue (hypodermis) is the deepest layer of skin, composed mainly of adipose tissue, connective tissue, blood vessels, and nerves. It lies beneath the dermis and functions in insulation, energy storage, and cushioning. It is...

Aripiprazole is an atypical (second-generation) antipsychotic that acts as a partial agonist at dopamine D2 and serotonin 5-HT1A receptors, and an antagonist at 5-HT2A receptors. It is used for schizophrenia, bipolar disorder, and as an...

Enterococcus spp. (E. faecalis, E. faecium) are gram-positive, catalase-negative, facultative anaerobic cocci in chains that are part of normal GI/GU flora but are important causes of nosocomial UTIs, endocarditis, and intra-abdominal in...

The femoral nerve is the largest branch of the lumbar plexus (L2-L4), providing motor innervation to the anterior thigh muscles (quadriceps, sartorius, iliacus) and sensory innervation to the anterior thigh and medial leg via the sapheno...

Endotracheal intubation is the placement of a flexible tube through the mouth (or nose) into the trachea to secure and maintain a patent airway, allowing mechanical ventilation and oxygenation. It is indicated for airway protection, resp...

Upper endoscopy (esophagogastroduodenoscopy, EGD) is a procedure using a flexible camera-equipped scope to visualize the esophagus, stomach, and duodenum. It is used for both diagnostic evaluation (e.g., GI bleeding, dysphagia, GERD comp...

Sucrose is a disaccharide composed of glucose and fructose linked via an α-1,2-glycosidic bond. It is hydrolyzed by sucrase (a brush-border enzyme) into its monosaccharide components for absorption. Sucrose intolerance due to sucrase-iso...

A continuous murmur is a heart murmur that begins in systole and extends without interruption through S2 into all or part of diastole, indicating a persistent pressure gradient between two chambers or vessels throughout the cardiac cycle...

Mesangial cells are specialized pericyte-like contractile cells located between glomerular capillary loops that provide structural support, regulate capillary blood flow via contraction, phagocytose trapped immune complexes and debris, a...

Human placental lactogen (hPL), also called human chorionic somatomammotropin (hCS), is a hormone produced by syncytiotrophoblast cells of the placenta. It promotes maternal insulin resistance and lipolysis to ensure adequate glucose and...

Dermatan sulfate is a glycosaminoglycan (GAG) composed of repeating disaccharide units of N-acetylgalactosamine and iduronic acid, found in skin, blood vessels, and heart valves. It is clinically significant in mucopolysaccharidoses (MPS...

Dengue virus is a mosquito-borne flavivirus (transmitted by Aedes aegypti/albopictus) causing dengue fever, characterized by high fever, retro-orbital headache, myalgias ('breakbone fever'), and rash. Severe forms include dengue hemorrha...

Hansen disease (leprosy) is a chronic granulomatous infection caused by Mycobacterium leprae (or M. lepromatosis), an acid-fast obligate intracellular bacillus that primarily affects skin and peripheral nerves. It is spread via prolonged...

A peripheral blood smear is a microscopic examination of a thin blood film stained (usually with Wright-Giemsa stain) to evaluate the morphology, number, and size of red blood cells, white blood cells, and platelets. It is a key diagnost...

Myelodysplastic syndrome (MDS) is a clonal hematopoietic stem cell disorder characterized by ineffective hematopoiesis, dysplastic blood cell morphology, and peripheral cytopenias despite a normal or hypercellular bone marrow. It primari...

CA 19-9 is a tumor marker primarily associated with pancreatic adenocarcinoma, used for monitoring disease progression and treatment response rather than screening or diagnosis. It can also be elevated in other GI malignancies (cholangio...

The therapeutic window (therapeutic index) is the dosage range between the minimum effective concentration and the minimum toxic concentration of a drug. A wide therapeutic window means a drug is generally safer, while a narrow one requi...

Gardner syndrome is a variant of Familial Adenomatous Polyposis (FAP) caused by an autosomal dominant mutation in the APC gene on chromosome 5q21. It is characterized by colonic adenomatous polyps along with extraintestinal manifestation...

Podocytes are specialized visceral epithelial cells of the glomerulus that wrap around glomerular capillaries with foot processes (pedicels), forming a key component of the filtration barrier. They maintain slit diaphragms that prevent p...

The anal canal is the terminal ~4 cm segment of the GI tract extending from the anorectal junction to the anus, marked by the dentate (pectinate) line dividing it into embryologically distinct upper and lower halves. It contains the inte...

Right shift refers to the presence of hypersegmented neutrophils (nuclei with 5 or more lobes) in the peripheral blood smear. This finding is classically associated with megaloblastic anemia due to vitamin B12 or folate deficiency. It is...

Hyaluronic acid (hyaluronan) is a non-sulfated glycosaminoglycan (GAG) composed of repeating disaccharide units of D-glucuronic acid and N-acetylglucosamine. It is a major component of the extracellular matrix, synovial fluid, and vitreo...

The Frank-Starling mechanism describes how the heart's stroke volume increases in response to an increase in venous return/end-diastolic volume (preload). This occurs due to increased stretch of cardiac myocytes, which optimizes actin-my...

Von Hippel-Lindau (VHL) disease is an autosomal dominant tumor suppressor syndrome caused by mutations in the VHL gene on chromosome 3p25-26, leading to constitutive activation of hypoxia-inducible factor (HIF) and increased angiogenic g...

Primary immunodeficiencies (PIDs) are a heterogeneous group of >450 genetic disorders that impair immune system development or function, leading to increased susceptibility to infections, autoimmunity, and malignancy. They are classified...

Adenosine deaminase (ADA) is an enzyme in the purine salvage pathway that converts adenosine and deoxyadenosine to inosine and deoxyinosine, respectively. ADA deficiency is a classic cause of severe combined immunodeficiency (SCID), the...

The red pulp is the portion of the spleen responsible for filtering aged, damaged, or abnormal red blood cells and recycling iron from hemoglobin. It contains splenic cords (of Billroth) and venous sinuses lined by specialized endotheliu...

ESBL (Extended-Spectrum Beta-Lactamase) refers to bacterial enzymes that confer resistance to penicillins, cephalosporins (including 3rd/4th generation), and aztreonam by hydrolyzing their beta-lactam ring. Commonly produced by Enterobac...

The radial nerve (C5-T1, posterior cord of brachial plexus) innervates the posterior arm and forearm extensor muscles, enabling extension of the elbow, wrist, and fingers, plus thumb abduction. It also provides sensory innervation to the...