A 50-year-old male underwent total gastrectomy for gastric cancer 5 years ago. Over the past year, he has progressively experienced dizziness, fatigue, palpitations, and shortness of breath on exertion. Laboratory tests show: RBC count 1.5×10<sup>12</sup>/L, hemoglobin 55g/L, WBC count 3.2×10<sup>9</sup>/L, platelets 65×10<sup>9</sup>/L, reticulocytes 0.001 (0.1%). MCV 129fl (129μm<sup>3</sup>), MCH 36pg, MCHC 34%. The most likely diagnosis is: