A 70-year-old woman, previously in good health, is found to have an asymptomatic monoclonal immunoglobulin peak on serum electrophoresis. Her physical examination is normal, as is her CBC, electrolytes, renal function and calcium level. A skeletal survey does not reveal any osteolytic lesions and a bone-marrow aspirate reveals 5% plasma cells. Which of the following is the most likely diagnosis?