A Relapsing Acute Tubulointerstitial Nephritis as an Expression of a Kappa Light Chain Multiple Myeloma: A Case Report

Introduction: Light chain-mediated acute tubulointerstitial nephritis (LCTIN) is a rare and underrecognized renal manifestation of plasma cell dyscrasias, including multiple myeloma. It presents as a dense interstitial inflammatory infiltrate involving polyclonal lymphocytes and plasma cells, often mimicking other forms of tubulointerstitial nephritis and delaying the correct diagnosis. Case Presentation: A 46-year-old man was initially managed as having drug-induced acute interstitial nephritis due to NSAID use, responding only transiently to steroids. Upon relapse with worsening kidney function, hypercalcemia, and systemic symptoms, a second kidney biopsy demonstrated again an intense tubulointerstitial infiltrate and κ-light chain proximal tubulopathy. Conclusion: This case illustrates that LCTIN can mimic relapsing interstitial nephritis. Early recognition and appropriate plasma cell-targeted therapy may significantly improve renal outcomes and guide clinical management.