**Core Concept**
Maple syrup urine disease (MSUD) is a metabolic disorder caused by the deficiency of an enzyme involved in the breakdown of certain amino acids. This disorder leads to the accumulation of branched-chain amino acids (BCAAs) and their toxic byproducts in the body.

**Why the Correct Answer is Right**
The enzyme deficient in MSUD is branched-chain alpha-keto acid dehydrogenase complex (BCKDC), also known as branched-chain alpha-keto acid dehydrogenase (BCKAD). This enzyme is responsible for the decarboxylation of branched-chain alpha-keto acids, which are byproducts of BCAA catabolism. The deficiency of BCKDC leads to the accumulation of these toxic compounds in the body, causing the characteristic sweet odor of the urine and other symptoms of the disease.

**Why Each Wrong Option is Incorrect**
* **Option A:** Glucose-6-phosphatase is deficient in von Gierke's disease, a glycogen storage disorder, not MSUD.
* **Option B:** Tyrosinemia type I is caused by a deficiency of fumarylacetoacetate hydrolase (FAH), not BCKDC.
* **Option C:** Phenylketonuria (PKU) is caused by a deficiency of phenylalanine hydroxylase (PAH), not BCKDC.

**Clinical Pearl / High-Yield Fact**
MSUD is a classic example of a metabolic disorder caused by a deficiency of an enzyme involved in amino acid catabolism. Early diagnosis and treatment are critical to prevent long-term neurological damage and other complications.

**Correct Answer: C. Branched-chain alpha-keto acid dehydrogenase complex (BCKDC).**