Webster's New World Medical Dictionary

primum non nocere
See 
first do no harm.
principal joints of the body
See 
joints of the
body, principal.
Prinzmetal angina
See 
angina, Prinzmetal.
prion
A small proteinaceous infectious disease-
causing agent that is believed to be the smallest
infectious particle. A prion is neither bacterial nor
fungal nor viral and contains no genetic material.
Prions have been held responsible for a number of
degenerative brain diseases, including mad cow dis-
ease, Creutzfeldt-Jakob disease, fatal familial insom-
nia, kuru, and an unusual form of hereditary
dementia known as Gertsmann-Straeussler-
Scheinker disease.
private mutation
A rare gene mutation that is
usually found only in a single family or a small pop-
ulation. A private mutation occurs and is passed to
a few family members, but not to future generations.
PRK
Photorefractive keratectomy.
p.r.n.
Pro re nata. Abbreviation meaning as
needed. See also Appendix A, “Prescription
Abbreviations.”
pro 1
Professional.
2
Prothrombin.
pro
- A prefix (from both Greek and Latin) with
many meanings, including before, in front of, pre-
ceding, on behalf of, in place of, and the same as.
pro time
Prothrombin time.
probability
The likelihood that something will
happen. For example, a probability of less than .05
indicates that the likelihood of something occurring
by chance alone is less than 5 in 100, or 5 percent.
This level of probability is usually taken as the level
of biologic significance, so a higher incidence may
be considered meaningful. Abbreviated p.
proband
The family member through whom a
family’s medical history comes to attention. For
example, a proband might be a baby with Down syn-
drome. The proband may also be called the index
case, propositus (if male), or proposita (if female).