You are answering a USMLE-style medical question. Question: A 55-year-old man presents with a 2-month history of an increasing sensation of fullness in the upper left side of his abdomen. He complains of increasing fatigue and dyspnea. He has no history of serious illness and takes no medications. His temperature is 36.7°C (98.1°F), pulse is 90/min, respiratory rate is 18/min, and blood pressure is 125/70 mm Hg. His conjunctivae are pale. The examination of the heart and lungs shows no abnormalities. The splenic margin is palpable 8 cm (3.1 in) below the costal margin. No abnormal lymph nodes are found. Laboratory studies show: Hemoglobin 8 g/dL Mean corpuscular volume 90 μm3 Leukocyte count 3,000/mm3 Platelet count 85,000/mm3 Peripheral blood smear shows small lymphocytes with cell membrane projections. Bone marrow aspiration is unsuccessful. Cell immunophenotyping is positive for CD25. Which of the following is the most effective pharmacotherapy at this time? Options: A) Cladribine B) Fludarabine C) Prednisone D) Thalidomide Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.