A 28-year-old man with a family history of progressive neurodegeneration presents with exercise intolerance, myalgia, and recurrent episodes of dark urine after exertion. Serum creatine kinase is 2400 IU/L. Plasma lactate is disproportionately elevated relative to pyruvate (lactate:pyruvate ratio = 35:1, normal <20:1). Which investigation would best confirm a defect in lactate dehydrogenase or impaired NAD+ regeneration in the TCA cycle?