A genetic disorder (which is inherited through recessive genes) in which the body is unable to break down the amino acid phenylalanine. The resulting buildup of phenylalanine in the body causes mental retardation, mental disturbances, eczema (a skin disorder) and light skin pigmentation. The symptoms may be avoided if the disorder is diagnosed early enough in the newborn so that the child may be fed a diet low in phenylalanine.