You are answering a USMLE-style medical question. Question: A 75-year-old man presents to the physician for his annual blood pressure visit and blood work. He has no complaints other than occasional knee pain. The past medical history includes hypertension. He has had knee osteoarthritis for several years and takes occasional acetaminophen for pain control. His diet is full of vegetables. He exercises every day. He has no history of smoking. He had a brother who died at the age of 84 due to hematologic malignancy. The temperature is 36.8℃ (98.2℉), and the blood pressure is 125/85 mm Hg. The physical examination shows no abnormalities other than a reduced range of motion and crepitus in both knees. The laboratory test results are as follows: Hemoglobin 15 g/dL Leukocyte count 58,000/mm3 Platelet count 250,000/mm3 Flow cytometry on the peripheral blood reveals an absolute lymphocyte count of 50,000/mm3. Which of the following is the most likely diagnosis? Options: A) Chronic lymphocytic leukemia (CLL) B) Monoclonal B cell lymphocytosis (MBL) C) Monoclonal gammopathy of undetermined significance (MGUS) D) Multiple myeloma (MM) Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.