A 28-year-old man with a family history of progressive neurodegeneration presents with exercise intolerance, muscle pain, and myoglobinuria after exertion. Serum ammonia is elevated at 120 µmol/L (normal <50). Resting lactate is normal, but lactate rises disproportionately with mild exercise. A muscle biopsy shows lipid accumulation but normal mitochondrial morphology. Which investigation best identifies the specific defect in the electron transport chain?