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This biopsy was taken from a 60-year-old African American female with a history of diabetes mellitus and hypertension, who presented with acute kidney injury and a serum creatinine of 5.8 mg/dl. Workup revealed an IgA kappa monoclonal band by serum protein electrophoresis. The biopsy shows numerous PAS-pale, focally fractured tubular casts with associated cellular reaction (Fig 1). These casts frequently stained positive for Congo red (Fig 2), and focally showed apple-green birefringence upon polarization. Otherwise, Congo red stain was diffusely negative within the glomeruli, interstitium, vessels, and cytoplasm of tubular epithelium. Immunofluorescence showed kappa light chain restriction of the casts...