To increase the specificity for amyloid Congo red stains should be examined by what type of microscopy?

• A. Bright-field
• B. Electron
• C. Polarizing
• D. Fluorescence

Answer: (C)

The key idea is that Congo red–stained amyloid becomes birefringent when viewed with polarized light. The orderly, beta-pleated fibril structure of amyloid aligns the bound dye so that light splits into two rays, producing an apple-green birefringence. This specific color under polarized light is a highly characteristic, confirmatory feature of amyloid.

Bright-field microscopy won’t reveal this birefringence, so it lacks the diagnostic specificity. Electron microscopy looks at ultrastructure and isn’t used to confirm Congo red staining. Fluorescence might show some signal, but it does not provide the distinctive birefringence that confirms amyloid. Therefore, polarized microscopy is the best choice for increasing specificity in Congo red–stained amyloid evaluation.