Which stain is used to demonstrate amyloid deposits in tissue sections with apple-green birefringence under polarized light?

• A. Congo red
• B. Giemsa
• C. PAS
• D. Gram

Answer: (A)

Detecting amyloid deposits relies on the Congo red stain because of its specific binding to amyloid fibrils. When sections stained with Congo red are examined under polarized light, the aligned amyloid-Congo red complex produces an unmistakable apple-green birefringence, a hallmark used to confirm amyloid deposition. This optical property arises from the beta-pleated sheet structure of amyloid, which interacts with the dye in a way that alters light as it passes through the polarized filter. Other stains listed have different targets: Giemsa is used for cellular and parasite detail, PAS highlights carbohydrates like glycogen and mucopolysaccharides, and Gram distinguishes bacteria by cell wall properties.