Which enzyme deficiency is responsible for phenylketonuria (PKU)?

Phenylketonuria (PKU) is primarily caused by a deficiency of the enzyme phenylalanine hydroxylase. This enzyme plays a critical role in the metabolism of the amino acid phenylalanine, converting it into tyrosine. When this enzyme is deficient or inactive, phenylalanine accumulates in the body, leading to various neurological issues and intellectual disabilities if not managed through dietary restrictions.

The other enzymes listed are involved in different pathways. Tyrosine transaminase is important for the conversion of tyrosine into other compounds but is not directly related to the pathological accumulation of phenylalanine seen in PKU. p-Hydroxyphenylpyruvic acid oxidase is involved in the metabolism of tyrosine metabolites, while homogentisic acid oxidase is linked to a different condition, alkaptonuria. Therefore, the specific deficiency of phenylalanine hydroxylase is the key factor in the diagnosis and management of PKU.