**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), leading to an accumulation of branched-chain amino acids (BCAAs) and their toxic byproducts in the blood and urine.

**Why the Correct Answer is Right**
In MSUD, the BCKDC enzyme is impaired, causing the branched-chain amino acids (leucine, isoleucine, and valine) to be metabolized into their corresponding alpha-keto acids, which then accumulate in the blood and urine. These alpha-keto acids are excreted in the urine, giving it a characteristic sweet odor reminiscent of maple syrup. The impaired metabolism of BCAAs leads to neurological symptoms, developmental delays, and other systemic complications.

**Why Each Wrong Option is Incorrect**
* **Option A:** Not a characteristic feature of MSUD, as phenylalanine is associated with Phenylketonuria (PKU).
* **Option B:** Not a correct answer, as histidine is an aromatic amino acid and not directly related to MSUD.
* **Option D:** Not a characteristic feature of MSUD, as glutamine is a non-essential amino acid and not directly related to the disorder.

**Clinical Pearl / High-Yield Fact**
MSUD is an autosomal recessive disorder, and patients often present with symptoms of vomiting, seizures, and developmental delays within the first few weeks of life. Early diagnosis and treatment are crucial to prevent long-term neurological damage.

**Correct Answer: C. Leucine, Isoleucine, and Valine**