A trophozoite approximately 12 μm in diameter is observed on a trichrome-stained fecal smear. Which organism is most likely?

• A. Chilomastix mesnili
• B. Giardia duodenalis
• C. Trichomonas hominis
• D. Dientamoeba fragilis

Answer: (D)

Trophozoite size and shape on a stained stool smear are key clues for identifying intestinal parasites. Dientamoeba fragilis has small trophozoites that are round to oval and about 5–12 μm in diameter. Seeing a trophozoite around 12 μm on a trichrome stain fits this species well, especially since D. fragilis trophozoites are typically non-flagellated and can appear as compact, spherical to oblong forms.

By contrast, Giardia duodenalis trophozoites are larger and elongated (pear-shaped) and often about 10–20 μm long with a characteristic two-nucleus “face” appearance when seen in fixed preparations. Trichomonas hominis trophozoites are slender, motile flagellates and tend to be elongated rather than round. Chilomastix mesnili trophozoites are also pear-shaped and more elongated with a cytostome visible, not simply round.

So the size range and rounded appearance described align best with Dientamoeba fragilis.