Multiple myeloma with extramedullary small bowel plasmacytoma presenting as a pyrexia of unknown origin: a case report

Multiple myeloma is a haematological malignancy arising from clonal plasma cell proliferation, typically presenting with hypercalcaemia, renal impairment, anaemia, and lytic bone lesions. Fever is usually due to infection secondary to immunoparesis; prolonged fever from the disease itself is rare. Extramedullary plasmacytomas are an uncommon manifestation. We present a rare case of small bowel extramedullary plasmacytoma coexisting with multiple myeloma. A 67-year-old man presented with pyrexia of unknown origin. Infectious, autoimmune, and lymphomatous aetiologies were excluded. Investigations showed an elevated ESR and anaemia with marked rouleaux formation. Serum free light chain assay confirmed a monoclonal gammopathy, and bone marrow biopsy demonstrated 30% plasma cell infiltration, establishing a diagnosis of multiple myeloma. A small bowel lesion identified during evaluation for abdominal pain was histologically confirmed as an extramedullary plasmacytoma. This case highlights a rare concurrence of small bowel extramedullary plasmacytoma with multiple myeloma, presenting atypically as pyrexia of unknown origin.