A 58 yr old lady comes to the doctor with a 2-month history of progressive fatigue and feeling “wobbly.” She has rheumatoid arthritis and is compliant with her treatment. Pulse is 101/min. Physical examination reveals decreased light touch and vibration sense on her lower extremities. Laboratory tests show hemoglobin of 9 g/dL, MCV of 110 fL, and elevated levels of serum methylmalonic acid and homocysteine. This patient’s anemia most likely results from one of the following?