Which staining method is used to identify amyloid in tissues?

• A. Congo red
• B. PAS
• C. Gram stain
• D. Hematoxylin

Answer: (A)

Amyloid identification relies on Congo red binding to the beta-pleated sheet structure of amyloid fibrils. When tissue stained with Congo red is examined under polarized light, amyloid deposits show an apple-green birefringence, which is a distinctive diagnostic feature. This specificity for amyloid makes Congo red the standard stain for confirming amyloid in tissue sections.

Other stains serve different purposes: PAS highlights carbohydrates and related substances and may stain some tissues that contain glycoproteins, but it is not specific for amyloid. Gram stain targets bacteria, and hematoxylin (as part of routine H&E) stains nuclei and general tissue morphology rather than identifying amyloid.