Which staining method for amyloid relies on polarization to confirm specificity?

• A. Congo red on polarized light
• B. Congo red under bright-field
• C. Cresyl violet
• D. Toluidine blue

Answer: (A)

The key idea is that a confirmatory sign for amyloid is a specific optical property that appears only with the right staining and viewing conditions. Congo red binds to the cross-β sheet structure of amyloid, and when the stained tissue is examined under polarized light, the Congo red–amyloid complex exhibits apple-green birefringence. That birefringence is highly characteristic of amyloid and helps distinguish it from other tissue elements that may take up the dye but don’t show this optical property.

Under bright-field microscopy, Congo red staining alone can be non-specific because other structures may also take up the dye and appear colored, so polarization is used to confirm the presence of amyloid. The other stains listed—Cresyl violet and Toluidine blue—are used for different tissue components and do not rely on polarized light to confirm amyloid.

So, Congo red on polarized light is the method that uses polarization to confirm specificity for amyloid.