A 3-year-old boy from rural Maharashtra presents with recurrent episodes of hypoglycemia, lethargy, and hepatomegaly. His mother reports that symptoms worsen during fasting or intercurrent illness. Serum glucose is 45 mg/dL during a fasting episode. Liver biopsy shows hepatic steatosis. Plasma carnitine levels are markedly reduced (0.2 mmol/L; normal 40–50 mmol/L). Urine organic acids show elevated dicarboxylic acids. What is the primary biochemical defect underlying this clinical presentation?