Which tissue is used as the control slide for Congo Red?

• A. Known amyloid slide
• B. Grey matter
• C. Infected tissue
• D. Spleen or liver

Answer: (A)

The key idea is using a positive control that confirms the Congo Red staining procedure works for detecting amyloid. Congo Red binds to amyloid and, when viewed with polarized light, shows apple-green birefringence. A tissue section known to contain amyloid provides a reliable positive control to verify both the dye binding and the polarization response. Other tissues like grey matter, infected tissue, or spleen/liver may not consistently contain amyloid, so they wouldn’t reliably demonstrate that the stain and microscope setup are functioning properly. Therefore, a known amyloid slide is the best control for Congo Red.