Phenylketonuria is a rare, recessive, hereditary disorder that manifests itself in infancy, and, if untreated, causes lasting brain damage by the age of six months. Its symptoms include severe mental retardation and other neurological disorders.

The disease is caused by an abnormal accumulation of the amino acid phenylalanine in the body. Phenylalanine is one of the essential amino acids and is present in all natural diets. Normally, part of the dietary phenylalanine is converted into another amino acid, tyrosine, by an enzyme secreted by the liver. In PKU victims a structural defect in this enzyme leads to the excessive accumulation of phenylalanine. The excessive phenylalanine apparently interferes with the metabolism of sugar (and possibly fat) in the brain, causing brain damage. The tyrosine deficiency associated with phenylketonuria also results in a decrease in production of melanins, the dark pigments of eyes, skin, and hair. Thus victims of phenylketonuria are typically blond, light-skinned, and blue-eyed.

Phenylketonuria is detected in newborn infants by means of routine urine tests. It is treated by giving the child a diet low in phenylalanine for a period of several years.

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