The counterstain used in this protocol is most likely which?

• A. Nuclear fast red
• B. Eosin
• C. Methylene blue
• D. Hematoxylin

Answer: (A)

A counterstain’s job is to add contrast after the main staining so you can see overall tissue architecture clearly. When the primary chromogen yields a brown color (as with DAB in many immunohistochemistry protocols), you want a counterstain that highlights nuclei without dulling or obscuring that brown signal. Nuclear fast red fits perfectly here because it stains nuclei a bright red/pink, providing strong, crisp contrast against the brown-stained areas. This makes it easier to assess localization and morphology without the counterstain overpowering the primary signal.

Eosin colors cytoplasm and connective tissue pink, which can wash out or compete with the brown color; methylene blue is a simpler stain not specifically optimized as a nuclear counterstain in this context; hematoxylin stains nuclei blue, which can sometimes blur separate colors and doesn’t offer the red-nuclear contrast that’s often preferred with brown chromogens. So the red nuclear counterstain is the best fit for maintaining clear distinction in this scenario.