A 66-year-old man has experienced fatigue, a 5-kg weight loss, night sweats, and abdominal discomfort for 10 months. On physical examination, he has marked splenomegaly; there is no lymphadenopathy. Laboratory studies show hemoglobin, 10.1 g/dL; hematocrit, 30.5%; MCV, 89 gm3; platelet count, 94,000/mm3; and WBC count, 14,750/mm3 with 55% segmented neutrophils, 9% bands, 20% lymphocytes, 8% monocytes, 4% metamyelocytes, 3% myelocytes, 1% eosinophils, and 2 nucleated RBCs per 100 WBCs. The peripheral blood smear also shows teardrop cells. The serum uric acid level is 12 mg/dL. A bone marrow biopsy specimen shows extensive marrow fibrosis and clusters of atypical megakaryocytes. Which of the following is most likely to account for the enlargement in this patient's spleen?