Which of the following amino acidurias is characterized by accumulation of branched-chain amino acids and their ketoacid metabolites, with a distinctive sweet-smelling urine?

Explanation

## Maple Syrup Urine Disease (MSUD) — Branched-Chain Amino Aciduria **Key Point:** MSUD is caused by deficiency of the **branched-chain α-ketoacid dehydrogenase (BCKDC) complex**, leading to accumulation of leucine, isoleucine, valine, and their corresponding α-ketoacids. ### Biochemical Pathophysiology The BCKDC is a mitochondrial enzyme complex (similar in structure to pyruvate dehydrogenase) that catalyzes oxidative decarboxylation of branched-chain α-ketoacids: - **Leucine** → α-ketoisocaproate - **Isoleucine** → α-keto-β-methylvalerate - **Valine** → α-ketoisovalerate When deficient, these amino acids and their ketoacids accumulate in blood and urine, causing: - **Sweet or "maple syrup" odor** in urine (due to α-ketoacids) - **Hypoglycemia and metabolic acidosis** (ketoacids are organic acids) - **Neurological crisis** in neonatal form: seizures, hypertonia, developmental delay, coma ### Clinical Presentation by Type | Type | Onset | Enzyme Activity | Severity | Prognosis | |------|-------|-----------------|----------|----------| | **Classic (Type IA)** | 3–7 days | <5% | Severe; acute metabolic decompensation | Death or severe disability without treatment | | **Intermediate (Type IB)** | 1–2 weeks | 5–20% | Moderate | Better with management | | **Intermittent (Type II)** | Variable | 20–50% | Mild; crisis during stress/infection | Good with dietary management | | **Thiamine-responsive (Type III)** | Variable | Responsive to B₁ | Mild to moderate | Excellent with thiamine supplementation | **High-Yield:** MSUD is screened in newborn screening programs. Diagnosis is confirmed by **elevated plasma branched-chain amino acids** and **elevated urine α-ketoacids** (detected by 2,4-dinitrophenylhydrazine test — produces yellow precipitate). ### Management 1. **Acute phase:** Discontinue protein; provide high-calorie glucose/lipid infusion; correct acidosis 2. **Chronic management:** **Branched-chain amino acid-restricted diet** (synthetic formula with minimal BCAA) 3. **Thiamine supplementation** (especially in Type III) 4. **Avoid catabolic stress** (infection, surgery, fasting) **Clinical Pearl:** The sweet odor in urine is pathognomonic and may be the first clinical clue in a neonate. Untreated MSUD causes severe neurological damage within days of birth. **Mnemonic:** **MSUD = Maple Syrup + Urine + Dehydrogenase deficiency** — remember the enzyme complex (BCKDC) is the same family as pyruvate dehydrogenase.