Q. A 59-year-old woman with a 10-year history of rheumatoid arthritis presents to her primary care provider complaining of a new onset of progressive fatigue, poor concentration, and an unusual rash over the past month. She has been managing her rheumatoid arthritis with ibuprofen and prednisone. Her diet mostly consists of corn and corn-based products due to financial constraints. On physical examination, the provider notes a well-demarcated, scaly rash on sun-exposed areas, particularly on the hands and neck. Neurologic examination reveals mild disorientation to time and place, and her tongue is notably bright red and swollen. Laboratory studies show mild normocytic, normochromic anemia. What vitamin deficiency is most likely responsible for this patient's symptoms?