A 3-year-old child from rural Maharashtra presents with failure to thrive, developmental delay, and recurrent infections. Serum lactate is elevated at 8 mmol/L (normal <2), and pyruvate is 0.5 mmol/L. Muscle biopsy shows ragged-red fibres on Gomori trichrome stain. Which investigation is most appropriate to confirm the suspected mitochondrial cytopathy involving the electron transport chain?