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…
phototoxicity An exaggerated response to ultraviolet radiation (sunlight) characterized by rapidly developing or severe sunburn. Phototoxicity may be associated with certain medications, including the antibiotic tetracycline.
phototherapy Treatment with light. For example, a newborn with jaundice may be put under special lights to help reduce the amount of bilirubin pigment in the skin.
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.
photorefractive keratectomy A kind of laser eye surgery that is designed to correct vision by changing the shape of the cornea, potentially eliminating 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…
photophobia Sunglasses may also help.
photodynamic therapy A form of treatment that uses a photosensitizing agent, administered by mouth or intravenously, which concentrates selectively 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, photodynamic therapy…
phosphorylation A biochemical process that involves the addition of phosphate to an organic compound. Examples include the addition of phosphate to glucose to produce glucose monophosphate 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.
phosphorus An essential element in the diet and a major component of bone.
phosphate A form of phosphoric acid that may bind to other organic chemicals to form a variety of compounds. For example, calcium phosphate makes bones and teeth hard. See also phosphorylation.
phocomelia A birth defect in which the hands and feet are attached to abbreviated arms and legs. The term comes from phoco (meaning “seal”) and melia (meaning “limb”), to indicate that a limb is like a seal’s flipper, as in exposure of the developing fetus to thalidomide. Phocomelia may also, in some cases, be genetic.
phobia An unreasonable sort of fear that can cause avoidance and panic. Phobias are a relatively common type of anxiety disorder. Phobias can be treated with cognitive behavioral therapy, using exposure and fear-reduction techniques. In many cases, antianxiety or antidepressant medication proves helpful, especially during the early stages of therapy.