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?

MGUS is vastly more common than multiple myeloma, occurring in 1% of the population over age 50. Patients with MGUS have smaller M components (usually < 30 g/L); no urinary Bence Jones protein; < 10% bone marrow clonal plasma cells, and no anemia, renal failure, lytic bone lesions, or hypercalcemia. About 25% of patients with MGUS will go on to develop multiple myeloma. This patient's normal investigations and percentage of marrow plasma cells makes the diagnosis MGUS rather than myeloma.