The substance stained red in Congo red–stained tissue is most likely which of the following?

• A. Amyloid
• B. Fibrin
• C. Lipofuscin
• D. Keratin

Answer: (A)

Congo red specifically binds to amyloid deposits because of their characteristic beta-pleated sheet structure. When tissue containing amyloid is stained with Congo red, the deposits appear red (pink) under ordinary light. A confirmatory sign is that, under polarized light, the Congo red–stained amyloid shows apple-green birefringence. The other substances listed do not have this Congo red binding pattern or the distinctive birefringence, so they wouldn’t appear red in this stain. Thus, red staining with Congo red most likely indicates amyloid.