A 45-year-old chronic alcoholic man presents to the emergency department with acute onset of confusion, ataxia, and ophthalmoplegia. Serum lactate is elevated at 8 mmol/L (normal: 0.5–2.2 mmol/L). The structure marked **C** in the Krebs cycle diagram (α-ketoglutarate dehydrogenase complex) is suspected to be dysfunctional. Which of the following cofactor deficiencies best explains the biochemical and clinical findings in this patient?