What is the role of the modified acid-fast stain in parasitology?

• A. To detect acid-fast oocysts of Cryptosporidium, Cyclospora, and Cystoisospora
• B. To detect Giardia
• C. To identify eggs of intestinal helminths
• D. To stain bacterial spores

Answer: (B)

The test is about identifying organisms whose oocysts resist decolorization by acid-alcohol. The modified acid-fast stain highlights these acid-fast oocysts, making them stand out in a stool smear. In parasitology, that means detecting Cryptosporidium, Cyclospora, and Cystoisospora (Isospora) oocysts, which retain the red/pink dye after mild decolorization and appear against a contrasting background. This staining approach is not used for Giardia, which is not acid-fast and is usually found with other stains or antigen tests. It also isn’t the method for visualizing helminth eggs or bacterial spores, which require different stains. So the role of the modified acid-fast stain is to detect acid-fast oocysts of Cryptosporidium, Cyclospora, and Cystoisospora.