phenylketonuria (fĕn´əlkēt´ənŏŏr´ēə) (PKU), inherited metabolic disorder caused by a deficiency in a specific enzyme (phenylalanine hydroxylase). The absence of this enzyme, a recessive trait, prevents the body from making use of phenylalanine, one of the amino acids in most protein-rich foods, and almost always leads to mental retardation and schizoid changes when phenylalanine levels rise; convulsions also commonly occur. Early diagnosis and treatment, which includes a carefully regulated low-phenylalanine diet begun during the first few weeks of life, may prevent serious mental deficiency. Positive improvement has been seen even when therapy is started in well-established cases. In 2007 the FDA approved the use of sapropterin dihydrochloride as a treatment. The drug can boost the ability of persons with low levels of phenylalanine hydroxylase to break down phenylalanine but will not help those who lack the enzyme. Most states have made the PKU blood or urine test mandatory for all newborn infants.