The Art of Deception: Multiple Myeloma Imitating Spondyloarthritis-A Case Report.

Rheumatic symptoms can be associated with malignancies. Although sacroiliitis is not a classic paraneoplastic syndrome, malignant infiltration of sacroiliac joints by multiple myeloma can mimic inflammatory rheumatologic disease. Multiple myeloma (MM), a plasma cell malignancy often presenting with bone pain, can involve axial skeletal structures, including the sacroiliac joints. We report a 40-year-old male with chronic low back pain and bilateral ankle enthesitis, unresponsive to nonsteroidal anti-inflammatory drugs (NSAIDs), along with systemic symptoms. Initial HLA-B27 positivity and imaging suggested Spondyloarthritis (SpA), but a poor treatment response prompted further evaluation. Serum protein electrophoresis, immunofixation, and bone marrow biopsy confirmed IgG-kappa MM. The coexistence of HLA-B27 positivity and sacroiliac joint changes mimicking axial spondyloarthritis poses a significant diagnostic pitfall, underscoring the need to consider multiple myeloma in patients with atypical features or a poor response to standard rheumatologic therapy.