A 48-year-old woman with proteinuria undergoes renal biopsy. Light microscopy shows mesangial expansion, and electron microscopy reveals randomly arranged fibrils measuring 12 to 24 nm in the mesangium and GBM. Staining with Congo red is negative. The most likely diagnosis is:
- A AA amyloidosis
- B Fibrillary glomerulonephritis ✓
- C AL amyloidosis
- D Diabetic nodular glomerulosclerosis
Correct answer: B. Fibrillary glomerulonephritis
Explanation
Fibrillary glomerulonephritis is characterized by random, nonbranching fibrils of 12 to 24 nm (thicker than amyloid fibrils, which measure 7 to 10 nm) composed of polyclonal IgG, and it is consistently Congo red negative. Both AA and AL amyloid show apple-green birefringence under polarized light with Congo red, excluding options A and C despite overlapping fibrillar ultrastructure. Diabetic nodular glomerulosclerosis has acellular Kimmelstiel-Wilson nodules without organized fibrils on electron microscopy.
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.