A 45-year-old man with chronic suppurative bronchiectasis develops progressive nephrotic-range proteinuria. Renal biopsy shows amorphous eosinophilic material expanding the mesangium and capillary walls. The special stain expected to confirm the diagnosis is:

• A Thioflavin T negativity with lambda light chain restriction on immunofluorescence
• B Silver methenamine stain showing spike formation along capillary loops
• C Masson trichrome showing fuchsinophilic subendothelial deposits
• D Congo red stain showing apple-green birefringence under polarized light ✓

Explanation

Renal amyloidosis appears as amorphous, hyaline, PAS-negative material in mesangium and vessel walls. Congo red staining gives apple-green birefringence under polarized light because amyloid fibrils have a crossed beta-pleated sheet structure. Chronic inflammatory disease such as bronchiectasis points to secondary (AA) amyloid derived from serum amyloid D protein. Spike pattern belongs to membranous nephropathy, and lambda restriction with Congo red negativity describes light chain deposition disease, not amyloid.

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.