Webster's New World Medical Dictionary

pneumonitis, radiation
Inflammation of the
lungs that results from radiation. Radiation pneu-
monitis typically occurs after radiation treatments
for cancer within the chest or breast. Radiation
pneumonitis usually manifests itself 2 weeks to 6
months after completion of radiation therapy.
Symptoms include shortness of breath upon activity,
cough, and fever. If radiation pneumonitis persists,
it can lead to scarring of the lungs, referred to as
radiation fibrosis. See also 
fibrosis, radiation.
pneumopericardium
Air or other gas in the sac
surrounding the heart (pericardium).
pneumothorax
Free air in the chest outside the
lung. Pneumothorax can occur spontaneously, fol-
low a fractured rib or other trauma, occur in the
wake of chest surgery, or be deliberately induced in
order to collapse the lung.
PNL
Percutaneous nephrolithotripsy.
PNS
Peripheral nervous system.
p.o.
Per os. Abbreviation meaning by mouth,
orally. See also Appendix A, “Prescription
Abbreviations.”
podiatrist
A specialist in the diagnosis and care
of foot disorders, including their medical and surgi-
cal treatment.
poikiloderma
Extra pigmentation of the skin
that demonstrates a variety of shades and is associ-
ated with widened capillaries (telangiectasia) in the
affected area.
poikiloderma congenita
See 
Rothmund
-
Thomson syndrome.
point, McBurney
See 
McBurney point.
point mutation
A single nucleotide base change
in DNA. For example, a point mutation is the cause
of sickle cell disease.
poison
Any substance that can cause severe
organ damage or death if ingested, breathed in, or
absorbed through the skin. Many substances that
normally cause no problems, including water and
most vitamins, can be poisonous if taken in exces-
sive quantity. Poison treatment depends on the
substance.