In phenylketonuria, the mutation affects which enzyme?

The correct answer is Option (2) → Phenylalanine hydroxylase

Phenylketonuria (PKU) is an inborn error of metabolism, inherited as an autosomal recessive trait. The person lacks the enzyme that converts the amino acid phenylalanine into tyrosine. Phenylalanine then builds up and is changed into phenylpyruvic acid and other derivatives, which damage the brain. The faulty gene is the one for phenylalanine hydroxylase. Tyrosine kinase and glucose isomerase are other enzymes and have no link with PKU.

NCERT Extract:
Pleiotropy – “… An example of this is the disease phenylketonuria, which occurs in humans. The disease is caused by mutation in the gene that codes for the enzyme phenyl alanine hydroxylase (single gene mutation). This manifests itself through phenotypic expression characterised by mental retardation and a reduction in hair and skin pigmentation.”
Mendelian Disorders – “… The affected individual lacks an enzyme that converts the amino acid phenylalanine into tyrosine. As a result of this phenylalanine is accumulated and converted into phenylpyruvic acid and other derivatives. Accumulation of these in brain results in mental retardation.”