A 58-year-old man with nephrotic-range proteinuria, restrictive cardiomyopathy, and macroglossia has a renal biopsy showing amorphous eosinophilic material in glomeruli and vessel walls. Serum free light chains show an excess of lambda over kappa with a ratio of 0.02. Staining of the biopsy with Congo red will demonstrate which finding under polarized light?

• A Bright yellow fluorescence after thioflavin-T only
• B Blue-green metachromasia
• C Apple-green birefringence ✓
• D Loss of staining with permanganate pretreatment

Explanation

Amyloid stained with Congo red shows orange-red coloration by ordinary light and characteristic apple-green birefringence under polarized microscopy, the diagnostic hallmark regardless of amyloid type. In AL amyloidosis the fibrils derive from monoclonal light chains, lambda slightly more often than kappa, matching the inverted ratio here. Thioflavin-T fluorescence exists but is not the classic polarized-light answer, and permanganate sensitivity distinguishes AA from AL amyloid, not Congo red appearance itself.

Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.