A 32-year-old woman with a 6-month history of recurrent sinusitis and bronchitis presents to the immunology clinic. Serum protein electrophoresis shows a monoclonal spike. Flow cytometry reveals CD19+ B cells producing only kappa light chains. Serum IgM levels are markedly elevated (2800 mg/dL; normal <230 mg/dL), while IgG and IgA are low-normal. A bone marrow biopsy confirms lymphoplasmacytic infiltration. The patient is counselled about her condition and started on observation. Six months later, she develops progressive cytopenias and hepatosplenomegaly. What is the most appropriate next step in management?