A 35-year-old woman presents to her family physician’s office with fatigue of at least 3 months’ duration. Her only explanation is that keeping up with her twin 4-year-olds really tires her out, and she does not even have enough energy to make nutritious meals for her and her husband. Vital signs and ECG are normal, but a third heart sound is heard with auscultation and she is pale. Blood results are as follows: Hb, 8 g/dL; hematocrit, 30%; MCV, 115 fL; WBC, 8000/μL; platelets, 200,000/μL. A deficiency of which of the following substances can most likely account for these findings?