A 50-year-old male presents with fatigue for 1 year and gingival bleeding for 1 week. Physical examination: no lymphadenopathy, spleen palpable 1 cm below the costal margin. Lab results: hemoglobin (Hb) 75 g/L, white blood cell (WBC) count 3×10<sup>9</sup>/L, platelets (PLT) 60×10<sup>9</sup>/L. Blood smear shows giant platelets and macrocytic red blood cells. Bone marrow reveals 7% myeloblasts, small megakaryocytes, and erythroid precursors with prominent megaloblastic changes. No response to androgen or vitamin B<sub>12</sub> therapy. The diagnosis is: