Multiple myeloma

A 62-year-old man comes to the clinic for a routine yearly examination. He has no acute complaints and says that he has felt well for the past year. However, his father recently passed away from multiple myeloma and he insists that he be tested for it. His medical history is noncontributory. He exercises for one hour each morning and eats a balanced diet. His temperature is 37.0°C (98.6°F), pulse is 81/min, respirations are 12/min, and blood pressure is 136/82 mm Hg. Physical examination shows a well-developed and well-nourished male who appears his stated age. Serum protein electrophoresis shows a small but significant serum monoclonal protein spike. Complete blood count and complete metabolic panels are both within normal limits. Review of systems is negative for bone pain, fatigue, or urinary symptoms. Which of the following is the most likely diagnosis?