Phenylketonuria is an autosomal recessive genetic disorder caused by a mutation in the phenylalanine hydroxylase gene, which leads to reduced enzyme activity and the accumulation of phenylalanine and its metabolites in the body, resulting in disease. Phenylketonuria is the most common congenital disorder of amino acid metabolism, clinically characterized by developmental delays in intelligence, light pigmentation of the skin and hair, and a mousey odor to the urine. It is often due to a deficiency of phenylalanine hydroxylase.