A 63-year-old woman experiences a burning sensation in her hands and feet. Two months ago, she had an episode of swelling with tenderness in the right leg, followed by dyspnea and right-sided chest pain. On physical examination, the spleen and liver now appear to be enlarged. CBC shows hemoglobin, 13.3 g/dL; hematocrit, 40.1%; MCV, 91 mm3; platelet count, 657,000/mm3; and WBC count, 17,400/mm3. The peripheral blood smear shows abnormally large platelets. Which of the following is the most likely diagnosis?