Fanconi syndrome due to light chain proximal tubulopathy in a patient with multiple myeloma.

Fanconi syndrome (FS) in an adult patient is an unusual finding and it merits thorough evaluation. Paraproteinemias are one of the common etiologies in adult FS and need to be ruled out. Among the various forms of renal involvement in multiple myeloma, light chain proximal tubulopathy (LCPT) is the rarest. Usually, it causes proximal tubular dysfunction which is characterized by intracytoplasmic deposition of crystallized, mostly kappa monoclonal light chains in proximal tubules; however, glomerular crystal deposition is unusual. Herein, we are presenting a patient with renal dysfunction and FS. On evaluation, she was found to have multiple myeloma and renal biopsy showed LCPT with extensive crystal deposition in the proximal tubular epithelium along with crystal deposition in the glomerular capillary endothelium. The treatment of the underlying multiple myeloma caused remission of the FS.