Which organism is described as round, approximately 25 μm in diameter with a central body and a thin ring of cytoplasm containing nuclei?

• A. Endolimax nana
• B. Entamoeba dispar
• C. Blastocystis hominis
• D. Iodamoeba bütschlii

Answer: (C)

The main idea here is recognizing the distinctive vacuolar form of Blastocystis hominis on a stained smear. This form is round and relatively large, about 25 μm in diameter, and features a large central body (a vacuole) with a thin rim of cytoplasm around it that contains nuclei. In this presentation, the peripheral cytoplasm houses the nuclei, giving a ring-like appearance around the central vacuole. This morphology is characteristic of Blastocystis in its vacuolar stage and sets it apart from other intestinal protozoa.

For comparison, Endolimax nana is much smaller and lacks a prominent central vacuole with a peripheral ring of nuclei. Entamoeba species typically show a single nucleus with peripheral chromatin and do not have this large central vacuole/ring configuration. Iodamoeba bütschlii features a prominent glycogen vacuole that pushes the nucleus to the edge, not a circular ring of nuclei around a central vacuole. Therefore the described round cell with a central body and a thin ring of cytoplasm containing nuclei best matches Blastocystis hominis.