Congo Red stained sections are observed with which microscope?

• A. Polarizing microscope
• B. Brightfield microscope
• C. Electron microscope
• D. Phase-contrast microscope

Answer: (A)

Congo red staining is used to detect amyloid by its birefringence under polarized light. When viewed with a polarizing microscope, amyloid fibrils bound with Congo red show an distinctive apple-green birefringence due to the organized, beta-pleated sheet structure. This optical property is best seen only with polarized light, not with standard brightfield illumination. Hence, the correct microscope is the polarizing microscope. Other methods like brightfield won't reveal the birefringence, and electron or phase-contrast microscopy isn't used to visualize Congo red–induced birefringence.