Congo red staining of amyloid shows what under polarized light?

• A. No birefringence
• B. Apple-green birefringence
• C. Blue birefringence
• D. Red fluorescence

Answer: (B)

Amyloid has a rigid beta-pleated sheet structure that binds Congo red in a way that makes the dye-amyloid complex optically anisotropic. When viewed with polarized light, this anisotropy produces birefringence, and the characteristic display is an apple-green interference color. This apple-green birefringence is a classic, highly specific clue for amyloid deposits in tissue sections stained with Congo red. Other possibilities don’t match the observed property under polarized light: no birefringence would mean no optical anisotropy, blue birefringence is not the typical appearance for Congo red–amyloid, and red fluorescence is not the expected signal in this context.