A 50-year-old woman presents with easy fatigability, a smooth sore tongue, numbness and tingling of the feet, and weakness of the legs. A complete blood count shows a megaloblastic anemia that is not reversed by folate therapy. Hemoglobin is 5.6 g/dL, WBC count is 5,100/mL, and platelets are 240,000/mL. This patient most likely has a deficiency of which of the following vitamins?