A 60 year-old man comes to the physician because of weakness and light-headedness for 3 months. His diet consists of a healthy balance of fruits, vegetables, whole grains, and lean chicken and fish. He has a 10-year history of chronic autoimmune (Hashimoto) thyroiditis treated with thyroxine. Physical examination shows pallor. Serum studies show increased concentrations of methylmalonic acid and total homocysteine. A peripheral blood smear shows megaloblastic anemia. Which of the following is the most likely cause of the findings in this patient?