You are answering a USMLE-style medical question. Question: A 68-year-old man presents with a 6-month history of worsening fatigue and weight loss. He is especially concerned about a recent episode in which he noticed a blurring of his vision and headache; he also notes that his “memory is not as good as it used to be,” which his wife confirms. Physical exam is notable only for pallor and general weakness. Laboratory studies reveal anemia as well as an elevated erythrocyte sedimentation rate. Serum protein electrophoresis (SPEP) reveals a sharp, narrow spike of monoclonal IgM; serum IgM concentration is 6.3 g/dL. What is the most likely diagnosis in this case? Options: A) IgM monoclonal gammopathy of undetermined significance (MGUS) B) Multiple myeloma C) Waldenstrom’s macroglobulinemia D) Non-Hodgkin’s lymphoma Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.