**Question:** Maple syrup urine is due to defective decarboxylation of

A. Valine
B. Isoleucine
C. Leucine
D. Phenylalanine

**Core Concept:** Maple syrup urine disease (MSUD) is a rare genetic disorder that affects the body's ability to break down certain amino acids. In this case, the defective enzyme is branched-chain alpha-keto acid decarboxylase (BCKD), which is responsible for decarboxylation of branched-chain amino acids (BCAAs). BCAAs include valine, isoleucine, leucine, and phenylalanine.

**Why the Correct Answer is Right:** C. Leucine is the correct answer because MSUD is caused by a deficiency in the BCKD enzyme, which is responsible for decarboxylation of branched-chain alpha-keto acids (BCKA). Leucine is one of the three BCAAs, along with valine and isoleucine, which means that a deficiency in BCKD can lead to an accumulation of leucine and its degradation products in the blood and urine, resulting in the characteristic maple syrup urine-like odor.

**Why Each Wrong Option is Incorrect:**

A. Valine: Valine is a BCAA, but it is not directly related to the defect in BCKD. The accumulation of valine is not the primary cause of MSUD, but rather, it is the result of the defective BCKD.

B. Isoleucine: Similar to Valine, isoleucine is a BCAA, but its accumulation is not the primary cause of MSUD. The deficiency in BCKD is the primary cause, resulting in the accumulation of isoleucine and its degradation products.

D. Phenylalanine: Phenylalanine is not a BCAA and is not directly related to the defect in BCKD. MSUD is caused by a deficiency in BCKD, which affects the decarboxylation of BCAAs, not phenylalanine.

**Clinical Pearl:** A comprehensive understanding of the molecular genetics and enzymology of MSUD is crucial for accurate diagnosis and management of this disorder. Early detection and intervention are essential to prevent severe intellectual disability and developmental delays in affected individuals.

**Correct Answer Explanation:**

In MSUD, the branched-chain alpha-keto acid dehydrogenase (BCKAD) complex deficiency leads to the accumulation of branched-chain alpha-keto acids (BCKA) and their corresponding branched-chain amino acids (BCAAs) in the blood and urine. These amino acids are derived from the intermediates of branched-chain amino acid catabolism.

**Why Each Wrong Option is Incorrect:**

A. Valine: Valine is a BCAA, and its accumulation is a result of the defective BCKD, not the direct cause of MSUD. The primary defect lies in the BCKAD complex, leading to the accumulation of BCAAs and BCKAs.

B. Isoleucine: Similar to Valine, isoleucine is a BCAA, and its accumulation is a