Kappa Light Chain-Restricted Multiple Myeloma with Biopsy-Proven Cast Nephropathy and Negative Bence–Jones Proteinuria: A Rare Clinical Presentation

Multiple myeloma (MM) is a malignant plasma cell disorder that commonly presents with anemia, bone pain, hypercalcemia, and renal impairment. We report the case of a 55-year-old male who presented primarily with acute kidney injury (AKI), pancytopenia, and systemic inflammation without the classical features of hypercalcemia or lytic bone lesions. Notably, urinary Bence-Jones proteins were negative despite markedly elevated serum kappa light chains and abnormal serum protein electrophoresis (SPEP). Bone marrow biopsy revealed modest plasmacytosis, while renal biopsy confirmed severe acute tubular injury with intratubular casts consistent with myeloma cast nephropathy. The patient required early initiation of hemodialysis and was subsequently managed with a bortezomib-based regimen, supportive care, and long-term planning for autologous stem cell transplantation (ASCT). This case underscores the importance of maintaining a high index of suspicion for MM in patients with unexplained AKI, highlights the indispensable role of renal biopsy in atypical presentations, and contributes to the growing recognition of variant clinical manifestations of this disease.