2,701 articles
Alveolar–arterial gradient
Alveolar–arterial gradient (A-a gradient, P(A–a)O2): difference between alveolar and arterial oxygen partial pressures, calculated via the alveolar gas equation; normal 5–15 mmHg, widens with V/Q mismatch, shunt, or diffusion limitation.
Alveolus
Alveolus is a terminal air sac found in the lungs, necessary for rapid gaseous exchange with blood. Approximately 300 million of these polyhedral sacs exist per adult lung.
Alzheimer's disease
Alzheimer's disease, a progressive neurodegenerative disorder defined by amyloid-beta plaques and tau tangles, is the most common dementia (60–80% of cases). Covers 2024 biological diagnostic criteria, p-tau217 blood tests, lecanemab and donanemab, ARIA risks, epidemiology, and contested amyloid hypothesis.
Amacrine cell
Amacrine cell: inhibitory retinal interneuron of the inner nuclear/ganglion cell layers, GABAergic or glycinergic, with 40–60+ types; key to direction selectivity, rod (AII) circuits, and conserved across 24 vertebrate species.
Ambidexterity
Ambidexterity: equal skill or no preference between both hands, distinct from mixed-handedness. Rare in humans (about 2% of adults); linked to weak lateralization, negligible cognitive differences, and no confirmed corpus callosum enlargement.
Amblyopia
Amblyopia, or lazy eye, is childhood vision loss from abnormal brain-eye development, with refractive, strabismic, and deprivational types; treated by patching, atropine, and newer binocular therapies.
Ambulatory blood pressure
Ambulatory blood pressure (ABPM): 24-hour cuff-based out-of-office blood pressure measurement during daily life. Reference standard for confirming hypertension; distinguishes white-coat from masked hypertension; daytime threshold 135/85 mmHg.
American Academy of Neurology
American Academy of Neurology (AAN), professional society of neurologists founded 1948 by A.B. Baker, headquartered in Minneapolis; 40,000+ members, publishes Neurology and Continuum, sets expert-witness and conduct standards, and advocates for brain health.
American Heart Association Distinguished Scientist Award
The American Heart Association Distinguished Scientist Award is the AHA's highest scientific honor, created in 2003 and given yearly to about six members for sustained cardiovascular research.
Amoebic keratitis
Amoebic keratitis, also called Acanthamoeba keratitis (AK), is a rare, severe corneal infection caused by free-living amoebae, mainly Acanthamoeba, marked by pain out of proportion to findings, ring infiltrates, contact lens and water exposure, and months of biguanide-diamidine therapy.
Amsler grid
Amsler grid: self-test chart of straight lines with a central dot, devised by Marc Amsler and marketed from 1947, used daily by AMD patients to detect metamorphopsia and central scotomas; high specificity, moderate sensitivity.
Amygdala
Amygdala: group of nuclei in the anterior medial temporal lobe central to fear conditioning, emotion recognition, and memory modulation; key divisions are the basolateral complex, central nucleus, and medial amygdala; landmark evidence from patient S.M. with Urbach-Wiethe disease.
Amygdala hijack
Amygdala hijack, also called emotional hijack, is Daniel Goleman's 1995 term for a sudden emotional takeover of rational thought by the amygdala; modern neuroscience rejects the fear-center mechanism it assumes.
Amyloid beta
Amyloid beta (Aβ, beta-amyloid) is a 36-43 amino acid peptide cleaved from APP by β- and γ-secretases; the main plaque component in Alzheimer's disease, a proposed innate immune antimicrobial peptide, and the target of lecanemab and donanemab.
Amyotrophic lateral sclerosis type 4
Amyotrophic lateral sclerosis type 4, or ALS4, is a rare inherited motor neuron disease caused by SETX mutations, beginning before age 25 and allowing a normal life span.
Anaesthesia of cornea
Anaesthesia of cornea (ICD-11 9A78.6, 角膜の知覚消失), loss of corneal sensation from trigeminal or corneal nerve damage; causes include herpetic keratitis, surgery, drugs, diabetes; leads to neurotrophic keratopathy.
Anal canal
The anal canal is the 2.5-4 cm terminal segment of the gastrointestinal tract between rectum and anus, divided by the dentate line, with involuntary internal and voluntary external sphincters governing continence.
Anastomosis
Anastomosis: a surgical connection between tubular structures such as blood vessels or intestine. Covers types (intestinal, vascular), end-to-end/side-to-side/end-to-side configurations, complications, history from Carrel to robotics.
Anatomical plane
Anatomical plane: imaginary flat surfaces (sagittal, coronal/frontal, transverse/axial, oblique) through the body in the anatomical position, used to divide the body into sections and standardize descriptions in anatomy, CT, MRI, and ultrasound.
Anatomical snuffbox
Anatomical snuffbox (radial fossa, radial foveola): triangular depression on the dorsum of the hand at the wrist, bounded by thumb extensor tendons, floored by the scaphoid, containing the radial artery; site of scaphoid fracture tenderness and distal radial access.
Anatomical terminology
Anatomical terminology: the standardized Latin-based nomenclature for human body structures, maintained by FIPAT of the IFAA, descended from the 1895 Basle Nomina Anatomica, currently the Terminologia Anatomica 2nd edition (2019).
Anatomical terms of location
Anatomical terms of location: standardized directional terms (anterior, medial, superior, proximal) defined against the anatomical position, governed by Terminologia Anatomica since 1895's Basiliensia Nomina Anatomica.
Anatomical terms of motion
Anatomical terms of motion: flexion, extension, abduction, adduction, rotation, circumduction, and special movements, defined relative to anatomical position and the sagittal, frontal, and transverse planes, standardized by FIPAT's Terminologia Anatomica.
Anatomical terms of muscle
Anatomical terms of muscle: standardized Latin and English muscle names in Terminologia Anatomica (TA, 1998; TA2, 2019), covering naming by shape, size, location, function, origin/insertion, and parallel versus pennate fiber architecture.
Andrea Natale
Andrea Natale, Italian-born American cardiac electrophysiologist, executive medical director of the Texas Cardiac Arrhythmia Institute in Austin; pioneer of atrial fibrillation catheter ablation, circumferential ultrasound vein ablation, epicardial RF ablation, and EPLive.
Angiocentric glioma
Angiocentric glioma is a rare WHO grade 1 pediatric-type diffuse low-grade glioma of cortex or brainstem causing refractory seizures, defined by MYB::QKI fusion and cured by resection.
Angiogenesis
Angiogenesis is new blood-vessel formation by sprouting or splitting of existing vessels, driven by hypoxia-HIF-VEGF signaling; it underlies development, exercise adaptation, tumors, and wet AMD, and is targeted by anti-VEGF drugs such as bevacizumab and ranibizumab.
Angiosarcoma
Angiosarcoma is a rare malignant tumor of blood and lymph vessel endothelial cells, about 1-2% of sarcomas, most common over age 70, often skin, breast, liver, or spleen; risk factors include lymphedema (Stewart-Treves syndrome) and prior radiation.
Angiosarcoma of skin
Cutaneous angiosarcoma is a rare, aggressive vascular sarcoma of the skin, mainly of the elderly head and scalp, or secondary to radiation or lymphedema; 5-year survival is roughly 30% and treatment is surgery plus radiotherapy.
Angiotensin
Angiotensin, especially angiotensin II, is the effector peptide of the renin-angiotensin system regulating blood pressure; discovered 1898-1958, targeted by ACE inhibitors and ARBs, whose comparative cardiovascular outcomes differ across trial populations.