Webster's New World Medical Dictionary

exonuclease
An enzyme that cleaves nucleotide
bases sequentially from the free ends of a nucleic
acid (DNA or RNA).
exophthalmos
A condition in which the patient
has protruding eyeballs, as in Graves disease.
exotropia
Divergent gaze. Also known as exter-
nal strabismus and, pejoratively, walleye.
expectorant
A medication that helps bring up
mucus and other material from the lungs, bronchi,
and trachea. An example of an expectorant is guaife-
nesin, which promotes drainage of mucus from the
lungs by thinning the mucus, and also lubricates the
irritated respiratory tract.
exposure
In cognitive behavioral therapy, the
process of exposing oneself to an event or a place
that causes anxiety or panic. The intention of con-
trolled exposure is to gradually lower the level of
stress and anxiety associated with the stimulus, to
eventually prevent panic attacks, obsessive-compul-
sive behaviors, and other unwanted reactions. See
also 
cognitive behavior therapy.
exposure and response prevention
A cogni-
tive behavior therapy technique that uses planned
exposures and exercises to reduce unwanted
responses. Abbreviated E&RP. See also 
cognitive
behavior therapy; exposure.
expression, gene
See 
gene expression.
expressivity
The consistency of a genetic dis-
ease. For example, Marfan syndrome shows vari-
able expressivity. Some persons with Marfan
syndrome merely have long fingers and toes, and
others have the full-blown disease, with dislocation
of the lens and dissecting aneurysm of the aorta.
expulsion, stage of
The second stage of labor,
lasting from the full dilation of the cervix until the
baby is completely out of the birth canal.