A 45-year-old woman with anemia for 1 year. Liver barely palpable below the costal margin, spleen not palpable. Hemoglobin 60 g/L, RBC 2.10×10<sup>12</sup>/L, WBC 3.2×10<sup>9</sup>/L (neutrophils 38%, lymphocytes 62%), no immature RBCs, platelets 50×10<sup>9</sup>/L. Bone marrow shows only 1 megakaryocyte per slide, elevated neutrophil alkaline phosphatase score, active bone marrow proliferation with predominantly late-stage granulocytic and erythroid series, and negative Ham test. The most likely diagnosis is: