A patient exhibits low activity of dehydrogenase enzymes that require NAD+ as a cofactor. Which vitamin deficiency is most likely contributing to impaired NAD+ synthesis, and what downstream metabolic process would be directly affected?

• A. Vitamin D deficiency is most likely; because vitamin D is a signaling molecule, it is required to activate mitochondrial oxidase enzymes and therefore NAD+ pools will be depleted, primarily affecting ketogenesis.
• B. Vitamin E deficiency is most likely; as the primary lipid-phase cofactor for dehydrogenases it will result in decreased NAD+ formation and impairment of anaerobic glycolysis exclusively.
• C. Vitamin C deficiency is most likely; impaired antioxidant protection will directly lower NAD+ synthesis and thus reduce fatty acid oxidation in peroxisomes specifically.
• D. Niacin (vitamin B3) deficiency is most likely; glycolysis and the TCA cycle dehydrogenases that rely on NAD+ would be impaired, reducing ATP production.

Answer: (D)