Crystalline κ light-chain proximal tubulopathy and Auer rod-like inclusions presenting as Fanconi syndrome: A case report

We report the case of a 53-year-old man who presented with Fanconi syndrome as the initial clinical manifestation. Laboratory findings included hypokalemia, hypophosphatemia, mild metabolic acidosis, and renal glucosuria. Kidney biopsy revealed crystalline light-chain proximal tubulopathy (LCPT) with κ-restricted light-chain deposition in proximal tubular epithelial cells. Bone marrow examination demonstrated multiple myeloma with plasma cells containing Auer rod-like intracytoplasmic inclusions, an extremely rare morphological finding. The coexistence of crystalline κ-restricted LCPT and Auer rod-like inclusions in plasma cells is exceedingly rare and remains poorly characterized. This case underscores the diagnostic value of kidney biopsy in unexplained Fanconi syndrome and suggests that abnormal physicochemical properties of monoclonal κ light chains may contribute to intracellular crystal formation in both renal tubular epithelial cells and plasma cells.