What stain shows green birefringence in amyloid deposits?

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

Answer: (A)

Green birefringence under polarized light is seen when amyloid deposits are stained with Congo red. The dye binds along the organized beta-pleated sheet structure of amyloid, and when viewed with polarized light, the Congo red–amyloid complex produces the distinctive apple‑green birefringence that confirms amyloid. Other stains behave differently: Thioflavin T binds to amyloid and fluoresces bright green under ultraviolet or blue light rather than creating birefringence, while PAS and Alcian Blue stain carbohydrates or acidic mucopolysaccharides and do not produce this characteristic birefringent signal.