A 38-year-old woman from rural Maharashtra presents with progressive fatigue, muscle weakness, and recurrent episodes of hypokalemia over 6 months. She reports a diet predominantly composed of refined carbohydrates and minimal protein intake. Laboratory investigations reveal: serum potassium 2.8 mEq/L, serum glucose 78 mg/dL (fasting), arterial pH 7.32, HCO₃⁻ 16 mEq/L, and elevated serum lactate 4.2 mmol/L (normal <2). Urinalysis shows ketonuria. A muscle biopsy shows lipid accumulation and mitochondrial abnormalities. Which enzyme defect in the TCA cycle would best explain the elevated lactate and metabolic acidosis in this patient?