A trichrome stain from a fecal smear reveals a parasite about 12 μm in diameter. Which organism is this most likely to be?

• A. Iodamoeba butschlii
• B. Entamoeba hartmanni
• C. Dientamoeba fragilis
• D. Endolimax nana

Answer: (C)

A main idea here is that the size of the parasite seen on a stained fecal smear helps identify which organism is present. A diameter of about 12 μm falls in the small-trophozoite range, which is characteristic for Dientamoeba fragilis. Its trophozoites are small, roughly 5–12 μm in diameter, and are commonly seen on trichrome-stained stools. This size fits best among the options provided when you’re looking at trophozoites on a trichrome stain.

Endolimax nana and Entamoeba hartmanni are even smaller in their trophozoite forms (roughly 4–8 μm and 6–10 μm, respectively), so they’re a less likely match for a 12 μm round parasite. Iodamoeba butschlii is more often associated with cysts that have a glycogen vacuole and can be larger, and its presentation on trichrome isn’t the same as the small trophozoites described here. So the 12 μm trophozoite best points to Dientamoeba fragilis.