Congo red staining is specific for detecting which extracellular deposits in tissue?

• A. Amyloid
• B. Hemosiderin
• C. Melanin
• D. Lipids

Answer: (A)

Congo red staining is used to detect amyloid, which are extracellular protein deposits that adopt a cross-beta sheet structure. The dye binds specifically to that beta-pleated sheet arrangement, giving a distinct red color in the deposits. When viewed with polarized light, the Congo red–amyloid complex shows apple-green birefringence, a diagnostic hallmark of amyloid. This combination of color under standard light and birefringence under polarized light helps distinguish amyloid from other tissue components. Hemosiderin, melanin, and lipids do not show this binding pattern or the characteristic birefringence with Congo red—hemosiderin is seen with Prussian blue, lipids with Sudan dyes, and melanin with specific pigment stains.