MAG and Paraproteinemia: A 72-year-old Caucasian man with a history of Waldenstrom’s macroglobulinemia, immunoglobulin M (IgM) paraproteinemia with high titer of myelin-associated glycoprotein (MAG) IgM antibody, and associated neuropathy was being effectively treated with maintenance rituximab and intravenous immunoglobulin (IVIG) when he was started on pembrolizumab for progression of metastatic urothelial carcinoma 31 months after surgery and adjuvant chemotherapy (gemcitabine, carboplatin, paclitaxel).