A 61-year-old man with nephrotic syndrome and cardiomyopathy undergoes kidney biopsy. Light microscopy shows amorphous eosinophilic material expanding the mesangium and vessel walls. The special stain expected to confirm the diagnosis produces which appearance?
- A Orange-red metachromatic staining after toluidine blue
- B Black staining of the deposits after silver methenamine stain
- C Apple-green birefringence under polarized light after Congo red stain ✓
- D Bright green fluorescence after fluoresceinated anti-IgG
Correct answer: C. Apple-green birefringence under polarized light after Congo red stain
Explanation
Renal amyloid appears on light microscopy as amorphous, nonargyrophilic eosinophilic material that stains with Congo red and shows characteristic apple-green birefringence under polarized light, reflecting the beta-pleated sheet configuration of amyloid fibrils. Silver methenamine highlights basement membranes and spikes in membranous nephropathy, and immunofluorescence is used for immune-complex diseases, neither of which confirms amyloid.
Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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