A 75-year-old woman lives alone and eats sparingly because of her low fixed retirement income. For the past 2 weeks, she has noticed pain in her right leg. On physical examination, there is marked tenderness to palpation over the lateral aspect of the right shin, a poorly healed cut on the right hand, and a diffuse hyperkeratotic skin rash. A radiograph shows a right tibial diaphyseal subperiosteal hematoma. Laboratory studies show a hemoglobin level of 11.3 g/dL. A deficiency of which of the following nutrients is most likely to explain these findings?