A 55-year-old man with long-standing ankylosing spondylitis develops nephrotic syndrome. Renal biopsy with Congo red staining shows apple-green birefringence under polarized light. Electron microscopy shows randomly arranged 8–12 nm fibrils. Which feature would distinguish AL from AA amyloidosis in this biopsy specimen?
- A Size of amyloid fibrils on electron microscopy (AL fibrils are larger)
- B Distribution: AL preferentially deposits in vessels and heart, while AA preferentially involves glomeruli
- C AA amyloid shows green birefringence while AL amyloid shows red birefringence under polarized light
- D Immunohistochemical staining with antibodies against SAA (positive in AA) versus antibodies against κ or λ light chains (positive in AL) ✓
Explanation
Fibril morphology on EM is identical for all amyloid types (haphazardly arranged 8–12 nm non-branching fibrils), so EM cannot distinguish AA from AL. The definitive method is immunohistochemistry using antibodies specific to the protein precursor: anti-SAA antibody stains AA amyloid, while anti-κ and anti-λ light chain antibodies identify AL amyloid. All types of amyloid stain the same with Congo red (apple-green birefringence under polarized light) because the staining property is a function of the β-pleated sheet secondary structure shared by all amyloid. In this case, the clinical context (ankylosing spondylitis) strongly suggests AA, but IHC confirms it.
Reference: Robbins & Cotran Pathologic Basis of Disease, 10th ed.
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Written and medically reviewed by the StethoPrep medical team.