Question: A 76-year-old man with hypertension and well-controlled diabetes presented to the hospital with poor appetite, abdominal distention, and intermittent nausea and vomiting for several weeks. He reported no fevers, sick contacts, or recent travel, but had an 18-pound unintentional weight loss during this time period with inability to tolerate solids for 4 weeks. Physical examination was notable for abdominal distension and tympany to percussion. Blood work showed a white blood cell count of 4,200/μL with a normal differential, hemoglobin of 13.1 g/dL, hematocrit of 38.2%, red blood cell count of 4.5 million/μL, and a platelet count of 239,000/μL. The metabolic profile was only notable for a potassium level of 2.8 mmol/L. There was no elevation in creatinine, calcium, or alkaline phosphatase. A computed tomography (CT) abdomen/pelvis scan without contrast showed multiple small bowel masses resulting in a small bowel obstruction, with the most proximal small bowel mass located in the third portion of the duodenum (Figure A). After NG tube decompression, enteroscopy was performed and identified an ulcerated mass near the ampulla measuring approximately 4 cm, and a single 1.5-cm ulcerated mass with adjacent edematous and friable mucosa in the third portion of the duodenum (Figure B). Duodenal biopsy showed small intestinal mucosa with prominent lymphoplasmacytic infiltrates in the lamina propria. Notably, these cells stained positive for CD138 on immunohistochemistry (Figure C). Serum protein electrophoresis and urine protein electrophoresis did not reveal any significant monoclonal spike or paraproteins. A subsequent bone marrow biopsy from the iliac crest was notable for an increased number of abnormal plasma cells, with less condensed nucleus and nucleolus. These abnormal plasma cells constituted 7% of the marrow (reference range, 0%–5%). The rest of the marrow differential consisted of the following: 63% myeloid cells (reference range, 30%–67%), 5% lymphocytes (reference range, 3%–17%), 25% erythroid cells (reference range, 10%–35%), and zero blasts (reference range, 0.3%–5%). There was no expression of kappa or lambda light chain. A subsequent whole-body positron emission tomography (PET)/CT showed no lytic lesions.