Webster's New World Medical Dictionary

pervasive developmental disorder
Abreviated
PDD, one of a group of disorders characterized by
delays in the development of multiple basic func-
tions including socialization and communication.
Parents may note symptoms as early as infancy and
typically onset is prior to 3 years of age. Symptoms
may include communication problems such as
using and understanding language; difficulty relat-
ing to people, objects, and events; unusual play with
toys and other objects; difficulty with changes in
routine or familiar surroundings, and repetitive
body movements or behavior patterns. Examples of
PDD include autism, Asperger’s syndrome, Rett’s
syndrome, and childhood disintegrative disorder.
Children with PDD vary widely in abilities, intelli-
gence, and behaviors. See also 
autism; Asperger’s
syndrome; Rett’s syndrome; childhood disinte-
grative disorder.
pes
Latin word meaning foot.
pes cavus
A foot with an arch that is too high.
pes planus
Flatfoot.
pest
See 
plague.
pestilence 1
Originally, bubonic plague. See also
plague.
2
Any epidemic disease that is highly con-
tagious, infectious, virulent, and devastating.
pestis
See 
plague.
PET scan
Positron emission tomography scan, a
highly specialized imaging technique that uses
short-lived radioactive substances. This technique
produces three-dimensional colored images. Unlike
computerized tomography (CT) and magnetic reso-
nance imaging (MRI), which look at anatomy or
body form, PET studies metabolic activity or body
function. PET scanning has been used primarily to
evaluate problems of the heart and nervous system
and to demonstrate the spread of cancer. The
radioisotope used in a PET scan is short-lived, and
the amount of radiation exposure the patient
receives is very low. Because the radiopharmaceuti-
cal contains a chemical that is commonly used by
the body, PET scanning enables the physician to see
the location of the metabolic process. For example,
glucose combined with a radioisotope shows where
glucose is being used in the brain, the heart muscle,
or a growing tumor, etc.
petechiae
Tiny red spots in the skin or mucous
membranes that do not blanch when pressed upon
and that result from blood leaking from capillaries.
The causes of petachiae include use of aspirin or
other medications, allergic reactions, autoimmune
disease, viral infection, thrombocythemia (an
abnormally high platelet level), idiopathic thrombo-
cytopenic purpura (ITP), leukemia and other bone
marrow malignancies that can lower the number of
platelets, chemotherapy and radiotherapy, and sep-
sis (bloodstream infection). Petechiae are often
seen right after birth in newborns and after violent
vomiting or coughing. A person with petechiae
should see a physician because they may be of
major consequence.