What stain is used for amyloid?

• A. Congo Red
• B. Crystal Violet
• C. Thioflavin T
• D. Alcian Blue

Answer: (A)

Staining for amyloid relies on the dye’s ability to bind the β-pleated sheet structure of amyloid fibrils. Congo red is the classic histochemical stain for this purpose. When tissue sections stained with Congo red are viewed under polarized light, the amyloid deposits show apple-green birefringence, a distinctive feature that helps confirm their presence. This diagnostic reliability is why Congo red is the stain of choice for identifying amyloid in routine practice.

Other stains mentioned don’t specifically highlight amyloid in standard light microscopy. Crystal violet is used in Gram staining and general cellular staining but isn’t selective for amyloid. Alcian blue targets acidic mucopolysaccharides, not amyloid. Thioflavin T can bind amyloid and is used in fluorescence studies, but it’s not the conventional light-m microscopy stain used for routine diagnosis.