The terminal ileum was removed from a 50-year-old woman during excision of a tumor. About 3 years later, the patient was admitted to the hospital. She is very pale. Hemoglobin is 9 g/dL, MCV (mean corpuscular volume) has increased to 110 μm3 (110 fL). The provisional diagnosis is a vitamin deficiency. What vitamin is the most likely one causing the symptoms and why?