A 3-year-old child has recurrent hypoglycemia, elevated serum ALT, and hyperammonemia after fasting. Liver biopsy shows microvesicular steatosis. Serum CK is normal. The enzyme defect MOST likely involves:

• A Glucose-6-phosphatase
• B Pyruvate carboxylase
• C Phosphoenolpyruvate carboxykinase (PEPCK)
• D Medium-chain acyl-CoA dehydrogenase (MCAD) ✓

Explanation

MCAD deficiency is the most common fatty acid oxidation defect; impaired beta-oxidation of medium-chain fatty acids causes microvesicular hepatic steatosis, fasting hypoglycemia (due to lack of acetyl-CoA for ketogenesis), and secondary hyperammonemia from accumulation of acylcarnitines that inhibit carbamoyl phosphate synthetase I. Glucose-6-phosphatase deficiency causes hepatomegaly but not hyperammonemia. Pyruvate carboxylase deficiency causes lactic acidosis.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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