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Aural polyp
Aural polyp (otic polyp, ICD-11 AB1A.0): a polypoid growth in the external ear canal or middle ear, usually inflamed granulation tissue from chronic otitis media or cholesteatoma, presenting with bloody drainage and hearing loss.
Auricle (anatomy)
Auricle (pinna): the visible outer-ear structure of elastic cartilage (lobule is fibrofatty) that funnels sound into the external acoustic meatus; landmarks helix, antihelix, concha, tragus; supplied by superficial temporal and posterior auricular arteries; develops from six hillocks of His.
Auscultation
Auscultation: listening to internal body sounds, mainly heart and lungs, with a stethoscope. Invented by Laennec in 1816; accuracy is modest (VHD sensitivity ~44% for GPs), now augmented by AI digital stethoscopes.
Autoimmune heart disease
Autoimmune heart disease is heart damage caused by the immune system attacking the body's own cardiac tissue, as organ-specific myocarditis or alongside systemic autoimmune diseases like lupus.
Autonomic dysreflexia
Autonomic dysreflexia, also called autonomic hyperreflexia, is a life-threatening condition in which a noxious stimulus below a spinal cord injury at or above T6 triggers sudden severe hypertension.
Autonomic nervous system
Autonomic nervous system (ANS): involuntary branch of the peripheral nervous system, divisions sympathetic, parasympathetic, and enteric; term coined by Langley in 1898; regulates heart rate, blood pressure, respiration, digestion.
Autonomic neuron
Autonomic neuron: efferent neuron of the autonomic nervous system, in preganglionic (CNS, cholinergic) and postganglionic (ganglionic, noradrenergic or cholinergic) forms, innervating smooth muscle, cardiac muscle, and glands.
Autosomal dominant childhood-onset proximal spinal muscular atrophy
Autosomal dominant childhood-onset proximal spinal muscular atrophy (SMALED, ORPHA:363447, OMIM 158600) is a rare genetic motor neuron disease with childhood-onset, proximal lower limb weakness, usually static or mildly progressive, caused mainly by DYNC1H1 or BICD2 mutations.
Autosomal dominant drusen
Autosomal dominant drusen (Doyne honeycomb retinal dystrophy, Malattia Leventinese, OMIM 126600) is a rare EFEMP1 R345W macular dystrophy with early-onset radial macular and peripapillary drusen, CNV risk, and anti-VEGF treatment.
Autosomal dominant spastic ataxia type 1
Autosomal dominant spastic ataxia type 1 (SPAX1, OMIM 108600) is a rare hereditary spastic ataxia from VAMP1 variants on 12p13, causing lower-limb spasticity, ataxia, dysarthria, and dysphagia with onset around ages 10 to 20.
Autosomal recessive cerebellar ataxia-epilepsy-intellectual disability syndrome due to RUBCN deficiency
SCAR15, or Salih ataxia (ORPHA:404499, OMIM 615705), is an extremely rare autosomal recessive cerebellar ataxia with epilepsy and intellectual disability caused by biallelic RUBCN variants, including an ancient Saudi founder frameshift mutation.
Autosomal recessive lower motor neuron disease with childhood onset
Autosomal recessive lower motor neuron disease with childhood onset, also DSMA4 (ORPHA:206580, OMIM 611067), is an ultra-rare PLEKHG5-related neuromuscular disease causing childhood proximal and distal weakness progressing to areflexic tetraplegia and respiratory failure.
Axial skeleton
Axial skeleton: the 80 central bones of the adult human skeleton (skull, hyoid, ear ossicles, vertebral column, ribs, sternum), distinct from the 126-bone appendicular skeleton of the limbs and girdles.
Axillary artery
Axillary artery: continuation of the subclavian artery from the first rib's outer border to the lower border of teres major, where it becomes the brachial artery; divided by pectoralis minor into three parts with six classic branches, highly variable in branching.
Axillary lymph nodes
Axillary lymph nodes: 20–30 nodes in the armpit in five groups (pectoral, lateral, central, subscapular, apical) draining the upper limb and breast; central to breast cancer staging and sentinel node biopsy.
Axillary nerve
Axillary nerve (circumflex nerve): C5-C6 branch of the posterior cord of the brachial plexus supplying deltoid and teres minor; passes through the quadrangular space; injuries cause shoulder abduction weakness.
Axis (anatomy)
Axis (C2, epistropheus): the second cervical vertebra, an atypical neck bone whose dens (odontoid process) is the pivot for head rotation and the main weight-bearing bone of the upper cervical spine.
Axon
Axon: the single output fiber of a neuron, carrying action potentials from the axon hillock to synaptic terminals; often myelinated with nodes of Ranvier; described by Cajal, who identified the growth cone (1890) and dynamic polarization (1891).
Axon guidance
Axon guidance: how growing axons navigate to targets via growth cone receptors reading attractive and repulsive cues (netrins, Slits, semaphorins, ephrins), Rho GTPase signaling, and roles in disease and regeneration.
Axon terminal
Axon terminal (presynaptic terminal, terminal bouton): swelling on an axon that releases neurotransmitter via calcium-triggered vesicle fusion at active zones; forms terminal or en passant synapses.
Azygos vein
The azygos vein is the unpaired vein of the right posterior thorax that drains the thoracic wall, arches over the right main bronchus, and empties into the superior vena cava.
Baby Fae
Baby Fae (Stephanie Fae Beauclair, 1984) was the first infant to receive a cross-species heart transplant, a baboon heart implanted by Leonard Bailey at Loma Linda; she lived 21 days.
Baculum
Baculum (os penis, penis bone): heterotopic bone in the penis of many placental mammals, notably carnivores, rodents, bats, and most primates but not humans; evolved independently at least nine times, lost at least ten times, used in systematics and sex identification.
Bainbridge reflex
Bainbridge reflex (atrial reflex): heart rate rises when venous return and atrial stretch increase, via vagal afferents to the medulla and vagal withdrawal plus sympathetic outflow. Described by F. A. Bainbridge in 1915 in dogs; weaker in humans; linked to respiratory sinus arrhythmia.
Banana bag
Banana bag: hospital slang for a 1-liter IV drip with thiamine 100 mg, folic acid 1 mg, magnesium 1-2 g, and a multivitamin, given for alcohol-related malnutrition; guidelines now favor higher-dose thiamine instead.
Bao Xishun (鲍喜顺)
Bao Xishun (鲍喜顺, Xi Shun), born 1951, is a Chinese herdsman from Chifeng, Inner Mongolia, measured at 2.361 m (7 ft 8.95 in); Guinness World Records' tallest living man 2005-2009, succeeded by Sultan Kösen.
BARI2D trial
BARI 2D (Bypass Angioplasty Revascularization Investigation 2 Diabetes), an NHLBI-funded 2x2 factorial randomized trial of 2,368 patients with type 2 diabetes and stable coronary disease, comparing revascularization versus medical therapy and insulin-sensitizing versus insulin-providing glycemic strategies.
Baroreceptor
Baroreceptor: stretch-sensing mechanoreceptor nerve ending in the carotid sinus, aortic arch, and cardiopulmonary circulation; fires via PIEZO, ENaC, ASIC, and TRP channels to the medullary baroreflex, buffering arterial blood pressure.
Barotrauma
Barotrauma: tissue injury from pressure differences across gas-filled body spaces during flying, diving, or hyperbaric therapy; ear barotrauma (airplane ear, otic barotrauma) is the most common form.
Bartholin's gland
Bartholin's gland (greater vestibular gland): one of two pea-sized vulvar glands, named for Caspar Bartholin the Younger, that secrete lubricating mucus; their ducts often form cysts and abscesses.