When PAS is used to demonstrate fungi, which counterstain may be used instead of hematoxylin?

• A. Fast Green
• B. Eosin
• C. Nuclear Fast Red
• D. Methylene Blue

Answer: (A)

PAS staining highlights fungal cell walls in magenta, so you want a counterstain that provides strong contrast without tinting the fungi themselves. Fast Green serves as a green counterstain that background-stains the tissue while leaving the magenta fungal elements visually distinct. This combination gives clear separation between the organisms and surrounding tissue, making identification easier. Hematoxylin is a common counterstain, but when you need an alternative, Fast Green is preferred because it enhances contrast without altering the color of the fungi.

Other countersaints would blur or compete with the magenta appearance: Eosin would add pink tones that can obscure the fungi; Nuclear Fast Red would color nuclei red and can distract from the fungal staining; Methylene Blue would color the background blue, which may reduce the sharpness of the magenta structures.