Congo red demonstrates which substance?

• A. Amyloid
• B. Bacteria
• C. Elastic fibers
• D. Nissl substance

Answer: (A)

Congo red demonstrates amyloid deposits. It binds to the beta-pleated sheet configuration that amyloid proteins adopt, and under polarized light these stained deposits show distinctive apple-green birefringence. That birefringence is the hallmark used to confirm amyloid in tissue. The other structures listed do not produce this specific birefringence with Congo red—bacteria are identified by other stains, elastic fibers are highlighted by stains like Verhoeff–Van Gieson or resorcin-fuchsin, and Nissl substance is seen with cresyl violet.