Which organisms have acid-fast oocysts detectable by the modified acid-fast stain?

• A. Ascaris lumbricoides, Trichuris trichiura, and Necator americanus
• B. Cryptosporidium, Cyclospora, and Cystoisospora
• C. Giardia, Entamoeba histolytica, and Giardia
• D. Taenia saginata, Diphyllobothrium latum, and Hymenolepis nana

Answer: (B)

Acid-fast staining in stool targets certain coccidian parasite oocysts that retain the dye after a milder decolorization used in the modified acid-fast procedure. The organisms whose oocysts are reliably acid-fast with this method are Cryptosporidium species, Cyclospora cayetanensis, and Cystoisospora belli. Their oocysts have cell walls that take up the stain and resist decolorization, making them stand out in the stained smear. The other organisms listed produce eggs or cysts that do not retain the acid-fast stain in this method, so they are not detected as acid-fast oocysts with the modified procedure.