You are answering a USMLE-style medical question. Question: A 72-year-old man comes to the physician because of a 2-month history of fatigue and worsening abdominal pain. During this period, he also has excessive night sweats and shortness of breath on exertion. Over the past 3 months, he has had a 5.6-kg (12-lb) weight loss. He had a myocardial infarction 3 years ago. He has hypertension, diabetes mellitus, and chronic bronchitis. His medications include insulin, aspirin, lisinopril, and an albuterol inhaler. He has smoked half a pack of cigarettes for the past 45 years. Vital signs are within normal limits. The spleen is palpated 6 cm below the left costal margin. Laboratory studies show: Hemoglobin 6.4 g/dL Mean corpuscular volume 85 μm3 Leukocyte count 5,200/mm3 Platelet count 96,000/mm3 A blood smear is shown. Bone marrow aspiration shows extensive fibrosis and a few scattered plasma cells. A JAK 2 assay is positive. Which of the following is the most appropriate next step in management?" Options: A) Cladribine B) Prednisone C) Imatinib D) Ruxolitinib Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.