Phenylketonuria is due to deficiency of –
**Core Concept**
Phenylketonuria (PKU) is a genetic disorder that affects amino acid metabolism. It is characterized by the inability to break down the amino acid phenylalanine (Phe), leading to its accumulation in the body. This condition is caused by a deficiency in the enzyme responsible for phenylalanine hydroxylation.
**Why the Correct Answer is Right**
The correct answer is **A. Phenylalanine hydroxylase**. This enzyme catalyzes the conversion of phenylalanine to tyrosine, a process that is essential for the normal metabolism of Phe. In individuals with PKU, a mutation in the gene encoding phenylalanine hydroxylase leads to its deficiency, resulting in the accumulation of toxic levels of Phe. This can cause a range of neurological and developmental problems if left untreated.
**Why Each Wrong Option is Incorrect**
* **Option B:** Tyrosine aminotransferase is an enzyme involved in the catabolism of tyrosine, not phenylalanine. Its deficiency would lead to a different set of metabolic problems.
* **Option C:** Histidine decarboxylase is an enzyme involved in the synthesis of histamine from histidine. It is not related to the metabolism of phenylalanine.
* **Option D:** Alanine transaminase is an enzyme involved in the transfer of amino groups between amino acids and keto acids. It is not directly involved in the metabolism of phenylalanine.
**Clinical Pearl / High-Yield Fact**
Phenylketonuria is a classic example of a genetic disorder that can be treated with dietary restrictions. If diagnosed early and treated with a strict diet, individuals with PKU can lead normal lives with minimal complications.
**Correct Answer:** A. Phenylalanine hydroxylase