Which stain is commonly used to demonstrate amyloid deposition in tissues?

• A. Congo Red
• B. Crystal Violet
• C. Colloidal Iron
• D. Thioflavin T

Answer: (A)

Amyloid has a beta-pleated sheet structure that Congo red binds to. When tissue sections stained with Congo red are viewed with polarized light, the amyloid deposits show an apple-green birefringence, a highly characteristic finding that confirms amyloid presence. This makes Congo red the classic stain for detecting amyloid in histology. Other stains serve different purposes—Crystal violet stains general cellular components, Colloidal iron highlights mucopolysaccharides rather than amyloid, and Thioflavin T can also bind amyloid and be used with fluorescence, but Congo red with polarized light remains the standard histochemical method.