Maple syrup urine disease (MSUD) results from deficiency of branched-chain alpha-keto acid dehydrogenase (BCKDH). Which three amino acids accumulate?

• A Phenylalanine, tyrosine, tryptophan
• B Lysine, arginine, histidine
• C Methionine, threonine, serine
• D Leucine, isoleucine, valine ✓

Explanation

BCKDH is the mitochondrial multienzyme complex that decarboxylates the alpha-keto acids derived from branched-chain amino acids (BCAAs): leucine, isoleucine, and valine. Deficiency causes accumulation of all three BCAAs and their corresponding keto acids (including alpha-ketoisocaproate from leucine). The burnt sugar or maple syrup odour of urine results from sotolone formed from the keto acids. Leucine is the most neurotoxic, causing cerebral oedema and encephalopathy in the neonatal period. Treatment involves dietary BCAA restriction and emergency management with glucose/insulin during metabolic crises.

Reference: Harper's Illustrated Biochemistry, 32nd ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.