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This image shows the prominent intracytoplasmic tubular epithelial crystals characteristic of a subset of light chain proximal tubulopathy cases associated with Fanconi syndrome (normoglycemic glycosuria, aminoaciduria, uricosuria, hyperphosphaturia with hypophosphatemia). The crystals represent precipitated nephrotoxic monoclonal light chains, and unlike cases without crystals, pronase-digested immunofluorescence may be helpful in proving light chain restriction when routine immunofluorescence is negative. Light chain proximal tubulopathy with crystals indicates an underlying clonal plasma cell proliferation, and may be seen in patients with monoclonal gammopathy of undetermined significance (MGUS), multiple myeloma, or lymphoma. Evidence of light chain cast nephropathy and/or monoclonal immunoglobulin deposition disease may also...