**Question:** "Classical phenyl ketonuria" is caused by deficiency of

A. Phenylalanine hydroxylase
B. Tyrosine hydroxylase
C. Cytochrome P450 enzymes
D. Alanine transaminase

**Correct Answer:** A. Phenylalanine hydroxylase

**Core Concept:** Phenylketonuria (PKU) is a rare genetic disorder characterized by the accumulation of phenylalanine (Phe) and its toxic derivatives in the body, leading to severe neurological damage if left untreated. PKU is caused by deficiency or dysfunction of enzymes involved in the phenylalanine catabolism pathway.

**Why the Correct Answer is Right:** Classical PKU is primarily caused by deficiency of Phenylalanine hydroxylase (PAH), an enzyme responsible for converting phenylalanine to tyrosine in the liver. PAH is part of the tetrahydrobiopterin (BH4)-dependent aromatic amino acid hydroxylase family, which includes tyrosine hydroxylase (TH), involved in the synthesis of catecholamines such as dopamine, norepinephrine, and epinephrine, and other enzymes in the pathway.

**Why Each Wrong Option is Incorrect:**

B. Tyrosine hydroxylase (TH) is incorrect because TH is involved in the synthesis of catecholamines (e.g., dopamine, norepinephrine, and epinephrine), not phenylalanine catabolism.

C. Cytochrome P450 enzymes are a group of enzymes involved in drug and xenobiotic detoxification, not phenylalanine catabolism.

D. Alanine transaminase (ALT) is an enzyme involved in amino acid catabolism, specifically the conversion of alanine to pyruvate. It has no role in phenylalanine catabolism.

**Clinical Pearl:** Early detection and management of PKU are crucial as it can lead to severe developmental and intellectual disabilities in affected individuals. Dietary intervention, such as restricting phenylalanine intake, can prevent neurological complications if initiated within the first few days of life.