Phenylketonuria is a recessive metabolic genetic disorder characterized by a mutation in the gene for the hepatic enzyme phenylalanine hydroxylase (PHA), making it nonfunctional. This enzyme catalyzes hydroxylation, which is a chemical process that introduces a hydroxyl group to an organic compound. When PHA activity is reduced, phenylalanine accumulates and is converted into phenylpyruvate, which is a mutated version of phenylalanine. When the phenylalanine is converted it is transported to the urine.