A 70-year-old woman presents with anemia. Imaging shows no significant lymphadenopathy or splenomegaly. An IgM kappa (3.4 g/dL) paraprotein is detected on SPEP/ IFE. Flow cytometry shows 20% monoclonal B cells expressing CD20 and surface Ig kappa, without CD5, CD10, or CD23. Molecular studies are positive for MYD88 L265P mutation. Which of the following is the most likely diagnosis?
