Webster's New World Medical Dictionary

oxyuris
A group of intestinal worms that includes
pinworm.
oz.
Abbreviation for ounce.
315
oz.
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p 1
In biochemistry, protein. For example, p53 is
a protein that has a mass of 53 kilodaltons.
2
In
population genetics, the frequency of the more
common of two different alternative (allelic) ver-
sions of a gene. (The frequency of the less common
allele is q.)
3
In statistics, probability.
p arm of a chromosome
The short arm of a
chromosome. Each human chromosome has two
arms—a short arm and a long arm. By interna-
tional convention, the short arm is termed p, and
the long arm is termed q. For example, if a gene is
on 4p12, that gene is on the short arm of chromo-
some 4, in region 12. See also 
chromosome.
PA 1
Physician assistant. 
2
Posteroanterior.
PA X
-
ray
An X-ray picture in which the beams
pass from back to front (posteroanterior), as
opposed to an AP (anteroposterior) film, in which
the rays pass through the body from front to back.
pacemaker
A device or system that sends electri-
cal impulses to the heart in order to set the heart
rhythm. A pacemaker can be the natural pacemaker
of the heart (the sinoatrial node) or it can be an
electronic device that serves as an artificial pace-
maker. See also 
pacemaker, artificial; pacemaker,
implantable; sinoatrial node.