A 68-year-old woman presents with fatigue and pallor for 3 months. Lab results: WBC 3.5×10<sup>9</sup>/L, RBC 1.5×10<sup>12</sup>/L, Hb 70g/L, PLT 87×10<sup>9</sup>/L. History reveals poor appetite, reduced food intake, and abdominal distension for months. Physical exam: pale-yellow complexion, no enlargement of superficial lymph nodes, liver, or spleen. Bone marrow aspiration shows active bone marrow proliferation with erythroid hyperplasia and megaloblastic changes, hypersegmented neutrophils, and large platelets. Gastroscopy reveals chronic superficial gastritis. The most likely diagnosis is: