Phenylketonuria, which is also called PKU, is an inherited condition in which the body cannot process a substance called phenylalanine. PKU is an inborn error of metabolism that can lead to severe mental retardation if it is not treated.

What is going on in the body?

Phenylketonuria is caused by an enzyme defect in the liver. Normally, the liver produces an enzyme that breaks down phenylalanine. Phenylalanine is a compound found in many foods. The liver of a person with phenylketonuria does not produce this enzyme. When this happens, phenylalanine and its by-products build up in the body. This leads to mental retardation.

What are the causes and risks of the disease?

A problem in a gene causes the enzyme defect that leads to phenylketonuria. In order to have PKU, a person must inherit the abnormal gene from both parents. Phenylketonuria occurs in 1 of 15,000 live births. It is much more common in whites than in other racial groups.