Phenylketonuria: The inherited inability to metabolize (process) the essential amino acid phenylalanine due to complete or near-complete deficiency of the enzyme phenylalanine hydroxylase.
Newborns are screened for phenylketonuria (PKU) by a blood test, usually with the Guthrie card bloodspot obtained from a heelprick.
Treatment is with a special diet low in phenylalanine. The goal is to normalize the levels of phenylalanine and tyrosine in the blood to prevent brain damage. Failure of treatment results in profound irreversible mental retardation, microcephaly (an abnormally small head), epilepsy, and behavior problems. It is clear that if the diet is not followed closely, especially during childhood, some impairment is inevitable. Maternal phenylketonuria requires a diet low in phenylalanine.
Phenylketonuria is inherited in an autosomal recessive manner, as are lesser degrees of phenylalanine hydroxylase deficiency. Phenylketonuria is abbreviated and commonly referred to as PKU.