Light chain-mediated acute tubulointerstitial nephritis as the sole manifestation of monoclonal gammopathy of renal significance in a young adult.

Light-chain-mediated acute tubulointerstitial nephritis (LC-ATIN) is an uncommon, under-recognised lesion within the spectrum of monoclonal gammopathy of renal significance (MGRS). It typically presents with acute kidney injury (AKI), minimal proteinuria and nonspecific symptoms, complicating diagnosis and risking progression to end-stage kidney disease. We report a man in his early 40s with unexplained AKI whose diagnosis was established after kidney biopsy demonstrating kappa-restricted tubular basement membrane deposits on immunofluorescence and electron microscopy. Bone marrow evaluation identified a small kappa-restricted plasma cell clone, confirming MGRS. He received clone-directed therapy followed by autologous stem cell transplantation (ASCT) to deepen haematological response and preserve renal function. Although ASCT may improve response depth in related MGRS entities, its benefit in LC-ATIN remains unproven. This case highlights the importance of serum free light chain testing, early biopsy and timely clone-directed therapy in unexplained AKI, including in younger patients.