A 42-year-old woman with a 10-year history of mitochondrial cytopathy presents with progressive muscle weakness, fatigue, and recurrent episodes of lactic acidosis triggered by infection or exercise. Her serum lactate is persistently elevated (6–8 mmol/L) despite supportive care. She has been trialed on multiple cofactors without sustained benefit. Which drug is the most appropriate choice to reduce lactate accumulation by enhancing pyruvate oxidation through the TCA cycle?