fĕnˌəlkētˌənoorˈēə (PKU), inherited metabolic disorder caused by the absence of 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; 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. Most states have made the PKU blood or urine test mandatory for all newborn infants.

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