A 3-year-old boy from rural Maharashtra presents with progressive weakness, developmental delay, and recurrent infections over 6 months. On examination, he has hepatomegaly, muscle hypotonia, and developmental quotient of 45. Serum lactate is elevated at 8 mmol/L (normal <2), and pyruvate is 0.4 mmol/L. Muscle biopsy shows ragged-red fibres on Gomori trichrome stain. MRI brain reveals basal ganglia involvement. Which complex of the electron transport chain is most likely defective in this patient?