In Phenylketonuria (PKU), what enzyme is deficient?

Phenylketonuria (PKU) is a genetic disorder caused by the deficiency of the enzyme phenylalanine hydroxylase. This enzyme is critical for the conversion of phenylalanine, an essential amino acid found in many protein-containing foods, into tyrosine, another amino acid that is important for protein synthesis and the production of neurotransmitters.

When phenylalanine hydroxylase is deficient, phenylalanine accumulates in the body, leading to its toxic effects on the brain and resulting in intellectual disabilities and other neurological issues. This is why early diagnosis and dietary management, which includes a low-phenylalanine diet, are crucial in managing PKU.

The other enzymes listed in the options are associated with different metabolic disorders. Homogentisic acid oxidase is related to alkaptonuria, tyrosinase is involved in the synthesis of melanin and its deficiency leads to albinism, and branched-chain alpha-keto acid dehydrogenase is associated with maple syrup urine disease. Each of these conditions involves a different metabolic pathway, reinforcing the specificity of phenylalanine hydroxylase's role in PKU.