Follow-up acid-fast staining of a fecal smear reveals round acid-fast–positive structures 8–10 μm in diameter. Which organism is suspected?

• A. Cryptosporidium
• B. Cyclospora
• C. Cystoisospora
• D. Microsporidia

Answer: (B)

Oocyst size and acid-fast staining pattern in stool are key clues used to distinguish intestinal protozoa. Cyclospora cayetanensis oocysts are typically round and measure about 8–10 μm in diameter, and they stain acid-fast (though the intensity can be variable). Cryptosporidium parvum oocysts are smaller, around 4–6 μm, and also acid-fast positive, but their smaller size makes them less likely in this scenario. Cystoisospora belli oocysts are much larger, usually 20–33 μm and ellipsoid, which does not match the observed 8–10 μm round form. Microsporidia are far smaller, with spores about 1–2 μm and not described by this acid-fast pattern. Therefore, the combination of an 8–10 μm round, acid-fast–positive oocyst most strongly points to Cyclospora cayetanensis.