Webster's New World Medical Dictionary

ARC
AIDS-related complex.
arch, aortic
See 
aortic arch.
archaea
A unique group of microorganisms that
are called bacteria (Archaeobacteria) but are genet-
ically 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).
arcus senilis
A cloudy opaque arc or circle
around the edge of the eye, often seen in the eyes of
the elderly.
ARDS
Acute respiratory distress syndrome.
areola 1
The small, darkened area around the
nipple of the breast.
2
The colored part of the iris
around the pupil of the eye.
3
Any small space in
a tissue.
arginine
An essential amino acid and a key com-
ponent of protein. Lack of arginine in the diet
impairs growth, and in adult males it decreases the
sperm count. Arginine is available in turkey,
chicken, and other meats, and as L-arginine in sup-
plements. Babies born without the enzyme phos-
phate synthetase have arginine deficiency syndrome;
adding arginine to their diets permits normal
growth and development.
argyria
Silver poisoning, resulting in ashen, gray,
discolored skin, and damage to other tissues of the
body. Caused by long-term use of silver salts or
other preparations containing silver.
arm
In popular usage, the appendage that
extends from the shoulder to the hand. However, the
medical definition refers to the upper extremity
extending from the shoulder only to the portion of
the elbow, excluding the forearm, which extends
from the elbow to the wrist. The arm contains one
bone: the humerus.