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First Reported Case of Extramedullary Plasmacytoma of the Appendix. An extramedullary plasmacytoma involving the gastrointestinal tract is extremely rare. We report an appendiceal extramedullary plasmacytoma in a 35-year-old man who presented to the emergency department because of upper abdominal pain. Computed tomography (CT) imaging revealed an incidental mass (3.7 × 1.9 × 1.6 cm) at the tip of the appendix. Microscopically, the appendix, periappendiceal soft tissue, and nearby lymph nodes were diffusely infiltrated by plasma cells that were kappa light chain restricted. Subsequent workup included an unremarkable bone marrow biopsy, as well as urine and serum electrophoresis. A diagnosis of kappa-restricted solitary extramedullary plasmacytoma was made. To our knowledge, this is the first case reported of an appendiceal extramedullary plasmacytoma in the medical literature.