A 45-year-old male with a 20-year history of alcohol abuse presents to the emergency department with acute onset ataxia, ophthalmoplegia, and confusion. Blood lactate is elevated at 8 mmol/L (normal <2). The structure marked **C** in the Krebs cycle diagram is inhibited due to deficiency of its essential cofactor. Which of the following best explains the accumulation of lactate in this patient?