Enzyme deficient in maple syrup urine disease:
**Core Concept**
Maple syrup urine disease (MSUD) is a metabolic disorder caused by a deficiency of the branched-chain alpha-keto acid dehydrogenase complex (BCKDC), which is essential for the breakdown of the branched-chain amino acids (BCAAs) leucine, isoleucine, and valine. This enzyme complex is crucial for the conversion of these amino acids into their respective alpha-keto acids, allowing them to enter the citric acid cycle.
**Why the Correct Answer is Right**
The correct answer is branched-chain alpha-keto acid dehydrogenase complex (BCKDC). This enzyme complex is a multi-subunit protein that consists of E1 alpha, E1 beta, E2, and E3 components. The E1 alpha subunit is responsible for the first step of the reaction, where it catalyzes the decarboxylation of the branched-chain amino acids. The deficiency of this enzyme complex leads to the accumulation of branched-chain amino acids and their toxic byproducts in the urine, resulting in the characteristic "maple syrup" odor.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as it does not relate to the metabolic disorder in question. The enzyme responsible for the breakdown of phenylalanine is phenylalanine hydroxylase, not related to MSUD.
**Option B:** This option is incorrect as it is the enzyme responsible for the breakdown of tyrosine, not the branched-chain amino acids. Tyrosine is broken down into fumarate and acetoacetate via the action of the enzyme tyrosine aminotransferase and the subsequent enzyme complexes.
**Option C:** This option is incorrect as it is the enzyme responsible for the breakdown of tryptophan, not the branched-chain amino acids. Tryptophan is broken down into alanine and acetoacetate via the action of the enzyme tryptophan 2,3-dioxygenase.
**Option D:** This option is incorrect as it is a general term for enzymes involved in the breakdown of amino acids, not specific to the branched-chain amino acids.
**Clinical Pearl / High-Yield Fact**
The characteristic "maple syrup" odor in MSUD is due to the accumulation of branched-chain amino acids and their toxic byproducts in the urine. This odor is often described as sweet and musty, and is a key diagnostic feature of the disease.
**Correct Answer:** C.