Webster's New World Medical Dictionary

MHC
Major histocompatability complex.
MI
Myocardial infarction. See 
heart attack.
micro-
Prefix meaning small, as in microcephaly
(small head) and microsomia (small body). The
opposite of micro- is macro-.
microangiopathy
A disease of the capillaries
(very small blood vessels), in which the capillary
walls become so thick and weak that they bleed,
leak protein, and slow the flow of blood. For exam-
ple, diabetes predisposes to the development of
microangiopathy in many areas, including the eye.
microbe
A minute organism typically visible
under a microscope. Microbes include bacteria,
fungi, and protozoan parasites.
metabolism, inborn error of
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microcephaly
An abnormally small head due to
failure of brain growth. Microcephaly is an ominous
sign because it is almost always associated with
developmental delay and mental retardation. Many
factors can impair the growth of the brain, includ-
ing intrauterine infections (such as rubella,
cytomegalovirus, and toxoplasmosis), intrauterine
chemical exposure (such as in fetal alcohol syn-
drome), excessive radiation exposure (as in an
atomic bomb explosion), chromosome abnormali-
ties (such as trisomy 13 and trisomy 18), and
genetic syndromes (such as Fanconi syndrome and
Williams syndrome). The opposite of microcephaly
is macrocephaly.
microcystic corneal dystrophy
See 
Cogan
corneal dystrophy.
microcytic
Literally, referring to any abnormally
small cell; in practice, referring to an abnormally
small red blood cell. For example, microcytic ane-
mia is characterized by small red blood cells. The
opposite of microcytic is macrocytic.
microdeletion
Loss of a tiny piece—a piece that
may be too small to be seen readily through a
microscope—from a chromosome. Microdeletions
can be detected via high-resolution chromosome
banding, molecular chromosome analysis (with
FISH), or DNA analysis. Disorders caused by
microdeletions include Angelman, DiGeorge,
Prader-Willi, and Williams syndromes.