Diagnose This (October 5, 2020)

What is at the top of your differential in a patient with a positive SPEP (IgG kappa) and the immunofluorescence finding below? (hint: lambda is negative)

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The immunofluorescence image depicts positive, amorphous, somewhat smudgy staining with kappa light chain predominately within the mesangium while lambda light chain is completely negative. This finding by immunofluorescence taken with the clinical history is highly suspicious for renal amyloidosis, AL-type. By light microscopy, numerous vessels and glomeruli contain PAS-pale amorphous material which is also non-argyrophilic on the silver stain. A Congo red stain in this case is positive showing characteristic apple-green birefringence under polarized light within this material. Electron microscopy demonstrated numerous randomly arranged fibrils around 10 nm in diameter within the mesangium and glomerular basement membranes consistent with amyloidosis.