Phenylketonuria (PKU), one of the most common inherited metabolic disorders, is estimated at 1 per 10,000 to 15,000 births. The inheritance is autosomal recessive. The mutant gene maps to chromosome 12. Deficient activity of the hepatic enzyme phenylalanine hydroxy-lase, which converts the phenylalanine to tyrosine, results in an accumulation of phenylalanine in the plasma and excretion of phenylpyruvic acid in the urine. Blood phenylalanine levels may reach 1,200 |imol/L or more. Acceptable levels range from 180 to 900 |imol/L. The urine test is positive for phenylpyruvic acid when the color of the urine changes to green after adding 10% FeCl3 to it. Early treatment with a phenylalanine-free diet prevents the development of neurologic complications. In untreated phenylketonuria, severe mental retardation, behavioral abnormalities, seizures, and motor dysfunctions are major neurologic complications. Patients usually have blond hair, fair skin, and blue eyes. Children of mothers with untreated PKU during pregnancy may have malformations such as microcephaly, agenesis of the corpus callosum, and cortical abnormalities.

A small brain, delayed and deficient myelination with spongiosis, and astrocytic fibrosis are characteristic pathologic features of untreated cases.

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