Biopsy of a reasonably well-demarcated mass of the nasopharynx from a 30 year old man, demonstrates a plasma cell proliferation. Serum electrophoresis shows a small monoclonal IgG spike. Bone marrow evaluation fails to demonstrate plasma cell proliferation and no lesions are seen on extensive skeletal x-rays. Which of the following is the most likely diagnosis?
Heavy chain disease Monoclonal gammopathy of undetermined significance Multiple myeloma Plasmacytoma