A 68-year-old man comes to his physician with complaints of severe fatigue and altered sensations in his extremities. Past medical history is remarkable for chronic gastritis. He drinks alcohol almost every day. His blood pressure is 130/80 mm Hg, heart rate is 95/min., respiratory rate - 14/min, and temperature of 37.1°C. His heart has a regular rate and rhythm, his lungs are clear to auscultation bilaterally. Neurologic examination reveals loss of touch and vibration sense in both upper and lower limbs. Laboratory investigation results include a hemoglobin of 80 g/L, Mean Corpuscular Volume (MCV) of 115 fL (the reference range is 80-100 fL) and White Blood Cells (WBC) of 3.0 × 109/L. Which of the following is the most likely diagnosis?