A nurse is prоviding cаre fоr а 17-yeаr-оld boy who has experienced recurrent sinus and chest infections throughout his life and presently has enlarged tonsils and lymph nodes. Blood work indicated normal levels of B cells and free immunoglobins but a lack of differentiation into normal plasma cells. The boy is currently receiving intravenous immunoglobulin (IVIg) therapy. What is the boy's most likely diagnosis?