Phenylketonuria I is due to deficiency of –
**Core Concept**
Phenylketonuria (PKU) is an autosomal recessive genetic disorder characterized by the inability to metabolize the amino acid phenylalanine (Phe). This leads to its accumulation in the blood and nervous system, causing intellectual disability and other neurological problems. The condition is caused by a deficiency in the enzyme phenylalanine hydroxylase (PAH).
**Why the Correct Answer is Right**
Phenylalanine hydroxylase is a crucial enzyme in the metabolism of phenylalanine. It catalyzes the conversion of phenylalanine to tyrosine in the liver. In the absence of this enzyme, phenylalanine accumulates in the blood and is metabolized to phenylpyruvic acid and other neurotoxic compounds. This leads to the characteristic symptoms of PKU, including intellectual disability, seizures, and behavioral problems. The deficiency of phenylalanine hydroxylase is caused by mutations in the PAH gene, which codes for the enzyme.
**Why Each Wrong Option is Incorrect**
**Option A:** (not specified) - This option cannot be evaluated without knowing the content.
**Option B:** Phenylalanine transaminase - This enzyme is involved in the metabolism of phenylalanine, but its deficiency does not cause PKU. Phenylalanine transaminase is responsible for the conversion of phenylalanine to phenylpyruvic acid, but this enzyme is not the primary cause of PKU.
**Option C:** Tyrosine hydroxylase - This enzyme is involved in the synthesis of catecholamines, but its deficiency is not related to PKU. Tyrosine hydroxylase is responsible for the conversion of tyrosine to L-DOPA, but this enzyme is not involved in the metabolism of phenylalanine.
**Clinical Pearl / High-Yield Fact**
PKU is a classic example of an autosomal recessive genetic disorder, where a deficiency of a single enzyme leads to a severe and debilitating condition. Early detection and treatment of PKU can prevent intellectual disability and other neurological problems.
**Correct Answer: D**