The disease phenylketonuria is caused by mutation in the gene that co…

The correct answer is Option (3) → Phenylalanine hydroxylase

Phenylketonuria is an inborn error of metabolism inherited as an autosomal recessive trait. The affected person lacks the enzyme that converts the amino acid phenylalanine into tyrosine, because the gene coding for phenylalanine hydroxylase is mutated. Phenylalanine then builds up and is turned into phenylpyruvic acid and other compounds, which harm the brain.

Option (1) Phenylalanine dehydrogenase — Incorrect. A dehydrogenase removes hydrogen; it is not the enzyme missing in PKU.

Option (2) Phenylalanine carboxylase — Incorrect. PKU is not about adding a carboxyl group; the missing step adds an –OH group.

Option (3) Phenylalanine hydroxylase — Correct. Phenylketonuria is an inborn error of metabolism inherited as an autosomal recessive trait. The affected person lacks the enzyme phenylalanine hydroxylase, which converts phenylalanine into tyrosine, so phenylalanine builds up.

Option (4) Phenylalanine hydrogenase — Incorrect. Adding hydrogen is not the step that fails in PKU.

NCERT Extract:
Pleiotropy – “We have so far seen the effect of a gene on a single phenotype or trait. There are however instances where a single gene can exhibit multiple phenotypic expression. Such a gene is called a pleiotropic gene. The underlying mechanism of pleiotropy in most cases is the effect of a gene on metabolic pathways which contribute towards different phenotypes. 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.”