A 35-year-old man from Bangalore with a 10-year history of recurrent myoclonic seizures and progressive cognitive decline is admitted with acute encephalopathy. Serum glucose is 92 mg/dL. CSF analysis shows elevated lactate (6 mmol/L, normal <2) and normal glucose. Brain MRI reveals bilateral striatal necrosis. Muscle biopsy shows ragged-red fibres. Genetic testing confirms heteroplasmic mtDNA mutations affecting Complex I. Which TCA cycle enzyme is MOST likely to be functionally impaired due to the reduced NAD+ regeneration from this mitochondrial defect?