[Proliferative Glomerulonephritis with Monoclonal Immunoglobulin Deposits (PGNMID) Associated with Marginal Zone B-cell Lymphoma: A Case of MGRS in a Patient with Nephrotic Syndrome and Rapidly Progressive Renal Failure].

Monoclonal gammopathies of renal significance constitute a heterogeneous spectrum of nephropathies characterised by renal deposition of monoclonal immunoglobulins or their fragments, produced by clonal proliferation of B lymphocytes or plasma cells, in the absence of diagnostic criteria for symptomatic multiple myeloma or overt lymphoma. In recent years, the growing recognition of these nosological entities has highlighted how early diagnosis and the implementation of clone-directed therapeutic strategies can significantly alter the natural history of the disease and improve long-term renal outcomes. Within this complex classification, proliferative glomerulonephritis with monoclonal immunoglobulin deposits is an exceptionally rare histological diagnosis, typically burdened by an unfavourable renal prognosis and a high risk of progression to end-stage renal disease. The peculiarity of this entity lies in the granular non organised deposition of monoclonal immunoglobulins at the glomerular level, which triggers an endocapillary and/or mesangial proliferative process with consequent impairment of filtration function. We present the clinical case of a 65-year-old male patient who developed overt nephrotic syndrome associated with rapidly progressive renal failure. Renal biopsy revealed PGNMID in the context of MGRS underlying marginal zone B-cell lymphoma. The patient underwent combination treatment with high-dose corticosteroids and rituximab, achieving an excellent clinical response characterised by complete remission of the nephrotic syndrome and normalisation of renal function. This case highlights the importance of a multidisciplinary diagnostic approach in MGRS and the potential benefit of clone-targeted therapies in the management of PGNMID, even in the presence of acute deterioration of renal function.