Abstract: This article reports a rare case of non-crystalline light chain proximal tubulopathy (LCPT). The patient, a 45-year-old female, presented with persistent bone pain, proteinuria, fractures following minor trauma, and renal insufficiency accompanied by Fanconi syndrome. Blood and urine immunofixation electrophoresis showed positive κ light chains, with a significantly elevated blood κ/λ ratio. Renal biopsy pathology revealed swollen proximal tubular epithelial cells containing various forms of non-crystalline inclusions, including disordered fiber bundles, ordered fine fibrillar structures, and low-density vacuole-like structures. Immunofluorescence and immunoelectron microscopy confirmed the κ light chain-restricted deposition within the inclusions, leading to a pathological diagnosis of non-crystalline LCPT, classified as monoclonal gammopathy of renal significance (MGRS). This case highlights the rare pathological features of LCPT, particularly the diversity of non-crystalline inclusions, and underscores the importance of combining light microscopy, immunofluorescence, and electron microscopy in the diagnostic process. It further deepens our understanding of MGRS and its renal pathological manifestations.

Key words: light chain proximal tubulopathy, monoclonal gammopathy of renal significance, Fanconi syndrome, non-crystalline inclusions