A 7-month-old infant becomes lethargic after an overnight fast during gastroenteritis. Blood glucose is 1.9 mmol/L (reference 3.5–6.0 mmol/L), beta-hydroxybutyrate is 0.2 mmol/L (reference 0.5–3.0 mmol/L during fasting), and plasma lactate is 5.8 mmol/L (reference 0.5–2.2 mmol/L). Acylcarnitine analysis shows increased medium-chain species. Which biochemical defect best explains the findings?
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