Webster's New World Medical Dictionary

phosphorus
An essential element in the diet and
a major component of bone.
phosphorylation
A biochemical process that
involves the addition of phosphate to an organic
compound. Examples include the addition of phos-
phate to glucose to produce glucose monophos-
phate and the addition of phosphate to adenosine
diphosphate (ADP) to form adenosine triphosphate
(ATP). Phosphorylation is carried out through the
action of enzymes known as phosphotransferases or
kinases.
photochemotherapy
See 
photodynamic therapy.
photodynamic therapy
A form of treatment that
uses a photosensitizing agent, administered by
mouth or intravenously, which concentrates selec-
tively in certain cells, followed by exposure of the
involved tissue to a special light (such as laser or
ultraviolet light), in order to destroy as much of the
abnormal tissue as possible. For example, photody-
namic therapy is used to treat some forms of cancer
and psoriasis. Also known as photochemotherapy.
photophobia
Painful oversensitivity to light. For
example, photophobia is often seen in measles and
iritis. Keeping lights dim and rooms darkened is
helpful when a patient has photophobia. Sunglasses
may also help.
photorefractive keratectomy
A kind of laser
eye surgery that is designed to correct vision by
changing the shape of the cornea, potentially elimi-
nating or reducing the need for glasses or contact
lenses. Abbreviated PRK. A laser is used to remove
the outer layer of the cornea and flatten the cornea.
PRK is done in a physician’s office, with anesthesia
via numbing eyedrops.
photosensitivity
Oversensitivity of skin to light.
Photosensitivity can be a side effect of medications
or result from diseases, such as lupus. Treatment
depends on the severity of the reaction and the
cause. Photosensitivity can be prevented by avoiding
skin exposure to ultraviolet light.
phototherapy
Treatment with light. For example,
a newborn with jaundice may be put under special
lights to help reduce the amount of bilirubin pig-
ment in the skin.
phototoxicity
An exaggerated response to ultra-
violet radiation (sunlight) characterized by rapidly
developing or severe sunburn. Phototoxicity may be
associated with certain medications, including the
antibiotic tetracycline.
phrenology
The study of variations in the size,
shape, and proportion of the cranium. Phrenology
was a pseudoscience of the 18th and 19th centuries,
based on the belief that a person’s character could
be learned by looking with care at the shape of the
person’s head and noting each and every bump and
depression in the skull. The individual mental facul-
ties were believed to be contained in neat compart-
ments in the cerebral cortex, and the sizes of these
faculties were supposed to be reflected by the con-
figuration of the skull.