A newborn presents with lethargy and urine with a burnt-sugar odor. What is the most likely enzyme deficiency?

• A. Debranching enzyme
• B. Carnitine palmitoyltransferase
• C. Branched-chain alpha-keto acid dehydrogenase
• D. Glucuronyl transferase

Answer: (C)

The clinical presentation described—a newborn with lethargy and urine that has a burnt-sugar odor—strongly suggests a condition associated with the metabolic processing of branched-chain amino acids, particularly maple syrup urine disease (MSUD). This disorder is linked to a deficiency in the branched-chain alpha-keto acid dehydrogenase complex, which is critical for the metabolism of branched-chain amino acids (leucine, isoleucine, and valine).

When the enzyme is deficient, the metabolic pathway is disrupted, leading to the accumulation of these amino acids and their corresponding keto acids in the blood and urine. The specific odor noted in the urine, reminiscent of burnt sugar, is a hallmark indicator of this condition. Newborns often present with nonspecific signs like lethargy, making prompt diagnosis and intervention crucial to prevent severe complications such as neurological damage.

The other options pertain to different metabolic disorders: the debranching enzyme deficiency is associated with glycogen storage disease type III, which presents differently, carnitine palmitoyltransferase deficiency relates to fatty acid oxidation disorders and typically presents with hypoketotic hypoglycemia and myopathy rather than the specific odor, and glucuronyl transferase deficiency (