Glomerular, Mesangial, and Tubular Cytoplasmic Fibrillary Inclusions in a Patient with Light-Chain Proximal Tubulopathy.

Monoclonal immunoglobulin or free light chains, produced in the setting of plasma cell dyscrasias, are a common cause of kidney injury with a wide variety of disease patterns. Light-chain proximal tubulopathy is a rare form of this disease that is often difficult to diagnose due to its relatively indolent presentation, subtle light microscopic findings, and often negative immunofluorescence using routine laboratory techniques. We report a case of light-chain proximal tubulopathy with cytoplasmic fibrillary inclusions in tubular cells, glomerular endothelial cells, and mesangial cells, which were positive for κ light chains on immunostaining after pronase digestion. Cytoplasmic fibrillary inclusions, composed of monoclonal protein, are strongly suggestive of underlying plasma cell dyscrasias, and such cases warrant further hematological investigations.