An inherited metabolic disease caused by a defect in the liver enzyme that prevents the conversion of the protein called phenylalanine into a useful form.

In phenylketonuria (PKU), a defect in the liver enzyme prevents the conversion of the protein called phenylalanine into a useful form. Instead, the phenylalanine builds up in the bloodstream and hampers normal brain development. If left untreated, a child with PKU will suffer mental retardation by the end of his or her first year, with the first signs of intellectual delay exhibited by six or seven months of age. Walking and talking are delayed; in some cases the child never accomplishes either. The retardation worsens until the age of six when brain growth is typically complete. One-third of untreated PKU children will suffer seizures.

Throughout most of the United States, a Guthrie PKU test is required for all newborns. It is...