Webster's New World Medical Dictionary

pyoderma gangrenosum
An ulcerating condi-
tion of skin that results in heaped borders with a
typical appearance. Pyoderma gangrenosum
appears to be mediated by the immune system, but
the exact cause is unknown. The lesion usually
begins as a soft nodule on the skin that then ulcer-
ates. The ulcer enlarges, and the skin at the edge is
purple-red. The ulcers can become quite large.
Pyoderma gangrenosum is associated with several
other diseases, including ulcerative colitis, Crohn’s
disease, rheumatoid arthritis, leukemia, and cryo-
globulinemia. Pyoderma gangrenosum is usually
responsive to corticosteroids.
pyogenic arthritis
Purulent arthritis with pus as
a result of infection within the joint. See 
arthritis,
septic.
pyrexia
See 
fever.
pyrimidine
One of the two classes of bases in
DNA and RNA. The pyrimidine bases in DNA are
thymine (T) and cytosine (C), and the pyrimidine
bases in RNA are thymine (T) and uracil (U). See
also 
purine.
pyuria
Pus in the urine. Pyuria is a sign of inflam-
mation, often related to infection.
purified protein derivative (PPD)
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q arm of a chromosome
The long arm of a
chromosome. All human chromosomes have 2
arms—a short arm and a long arm. By interna-
tional convention, the short arm is termed p, and
the long arm of the chromosome is termed q. For
example, if a gene is on 3q12, that gene is on chro-
mosome 3, on its long arm, in region 12.
Q fever
An infectious disease due to the bac-
terium Coxiella burnetii whose symptoms include
fever, headache, malaise, and pneumonia (intersti-
tial pneumonitis), but not rash. The Q stands for
query because the cause of the disease was long a
question mark. Q fever is a zoonotic disease and is
contracted from cattle, sheep, and goats. Chronic Q
fever (infection that persists for more than 6
months) may develop years after the initial infection
and can lead to serious complications including
endocarditis. Transplant recipients, patients with
cancer, and patients with chronic kidney disease are
at increased risk of developing chronic Q fever.