archaea A unique group of microorganisms that are called bacteria (Archaeobacteria) but are genetically and metabolically different from all other known bacteria. They appear to be living fossils, the survivors of an ancient group of organisms that bridged the gap in evolution between bacteria and multicellular organisms (eukaryotes).
arch, aortic See aortic arch.
ARC AIDS-related complex.
arbovirus A type of virus transmitted to humans by mosquitoes and ticks. Arbovirus can cause inflammation of the brain (encephalitis). The types of arboviral encephalitis that occur in the US include LaCrosse, eastern equine, western equine, and St. Louis encephalitis, all of which are transmitted by mosquitoes. Another arbovirus, Powassan, transmitted by ticks, is a…
arbitration agreement An arrangement in which the patient waives the right to sue the physician and, instead, agrees to submit any dispute to arbitration. Arbitration agreements are legal and binding. The arguments in their favor are that, for patients, the case can be settled faster, and more money can go to the patient (rather than…
arachnophobia An abnormal and persistent fear of spiders. Sufferers from arachnophobia experience undue anxiety, even though they realize that the risk of encountering a spider and being harmed by it is small or nonexistent. They may avoid going barefoot and may be especially alert when taking showers or getting into and out of bed.
arachnodactyly A condition in which a person has long, spider-like fingers and toes. Arachnodactyly is a frequent finding in those with Marfan syndrome. See also Marfan syndrome.
aqueduct of the midbrain See aqueduct of Sylvius.
aqueduct of Sylvius A canal between the third and fourth ventricles in the brain within the system of four communicating cavities that are continuous with the central canal of the spinal cord. The ventricles are filled with cerebrospinal fluid, which is carried by the aqueduct of Sylvius.
aqueduct A channel for the passage of fluid.
apthous ulcer See canker sore.
APS Antiphospholipid syndrome.