Which organism is described by a classic form with a large central body and peripheral nuclei, typically 6-40 μm in diameter?

• A. Entamoeba histolytica
• B. Lodamoeba biltschlii
• C. Blastocystis hominis
• D. Dientamoeba fragilis

Answer: (C)

The description matches Blastocystis hominis, which is well known for a vacuolar, or “classic,” form in which a large central body (a central vacuole) occupies much of the cell, and a thin rim of cytoplasm around the edge contains the nuclei, giving the impression of peripheral nuclei. The size range given, about 6–40 μm in diameter, fits the vacuolar form seen in many stool preparations. This distinctive morphology helps separate it from other intestinal protozoa.

Entamoeba histolytica usually shows a trophozoite with a single nucleus (and a central karyosome) and, in cysts, multiple nuclei oriented differently, not a large central vacuole with peripheral nuclei. Dientamoeba fragilis trophozoites are typically binucleate but lack a prominent central vacuole with a peripheral rim of nuclei. Lodamoeba biltschlii has different, smaller, and characteristically different forms.