Which description best fits Cystoisospora belli oocysts?

• A. 8–10 μm, variably acid-fast
• B. 4–6 μm, non-acid-fast
• C. 20–30 μm, ellipsoidal and strongly acid-fast
• D. 12–16 μm, spherical and weakly acid-fast

Answer: (C)

Cystoisospora belli oocysts are large and easily distinguished by their shape and staining. They are elongated or ellipsoidal, measuring roughly 20–30 micrometers in length, which sets them apart from the smaller, spherical oocysts of other intestinal coccidia. When stained with a modified acid-fast stain, these oocysts show strong acid-fast positivity, staining prominently red/pink. This combination of large size, ellipsoidal form, and strong acid-fast staining is the hallmark that best fits Cystoisospora belli, helping to separate it from organisms like Cryptosporidium (smaller, 4–6 μm, spherical) and Cyclospora (about 7–10 μm, variably acid-fast).