An iodine-stained fecal wet mount demonstrates a round structure about 25 μm in diameter with a homogenously stained central body surrounded by a thin ring of cytoplasm containing a number of nuclei. This best describes which organism?

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

Answer: (A)

The key idea here is recognizing a distinctive protozoan cyst appearance on an iodine-stained fecal smear: a round organism with a very large central vacuole-like area and a thin peripheral rim of cytoplasm that contains several nuclei. This configuration—central vacuole with a ring of peripheral cytoplasm studded with multiple nuclei and an overall size around 20–30 μm—is characteristic of Blastocystis species, often referred to historically as Blastocystis hominis.

Other parasites don’t fit this pattern. Endolimax nana tends to have a small, irregular nucleus and lacks a prominent central vacuole with a multi-nuclear peripheral rim. Entamoeba dispar cysts can have multiple nuclei, but their nuclei arrangement and overall cyst morphology differ from a large central vacuole with a peripheral, multi-nucleated ring. Iodamoeba bütschlii cysts usually show a large glycogen-containing body with a single nucleus rather than a peripheral ring of multiple nuclei. Therefore, the described structure best corresponds to Blastocystis hominis.