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Monoclonal gammopathy of renal significance (MGRS) refers to a spectrum of kidney disorders caused by monoclonal immunoglobulins produced by small, often clinically silent clones of B-cells or plasma cells that do not meet the diagnostic criteria for overt haematological malignancy. These entities are frequently overlooked, as their clinical presentation often mimics more common renal diseases and typical markers of monoclonal protein may be absent. We describe the case of a 49-year-old woman with type 2 diabetes and hypertension who presented with nephrotic-range proteinuria. Despite unremarkable findings on serum and urine protein electrophoresis and a normal bone marrow biopsy, serum free light chain analysis revealed significantly elevated kappa chains. Renal biopsy demonstrated classic features of both kappa light chain cast nephropathy and light chain deposition disease. This case highlights the role of renal biopsy and serum free light chain assays in the diagnosis of MGRS, especially when traditional paraprotein studies are inconclusive.