In Maple syrup urine disease, which of the following compounds is accumulated:
**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). This enzyme is responsible for the breakdown of the branched-chain amino acids (BCAAs) leucine, isoleucine, and valine. In MSUD, the accumulation of these amino acids and their toxic byproducts leads to a range of clinical symptoms.
**Why the Correct Answer is Right**
The correct answer is the branched-chain amino acid that accumulates in MSUD. Leucine is metabolized through the tricarboxylic acid (TCA) cycle, but in MSUD, the accumulation of leucine and its alpha-keto acid, alpha-ketoisocaproate (KIC), inhibits the BCKDC, leading to a vicious cycle of accumulation. The other options are also BCAAs, but they are not primarily responsible for the accumulation in MSUD.
**Why Each Wrong Option is Incorrect**
**Option A:** Isoleucine is also a BCAA, but it is not primarily responsible for the accumulation in MSUD.
**Option B:** Valine is a BCAA, but it is not the primary compound accumulated in MSUD.
**Option D:** This option is incomplete, but assuming it's another BCAA, it's still not the correct answer.
**Clinical Pearl / High-Yield Fact**
MSUD is an autosomal recessive disorder, and newborn screening programs often detect it through the measurement of blood or urine levels of the BCAAs.
**Correct Answer:** C.