A trichrome stain of a fecal smear shows a parasite about 20 × 15 μm in size; the trophozoite is most likely which organism?

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

Answer: (B)

Giardia duodenalis trophozoites are classically pear-shaped and reach about 10–20 μm in length, with a width around 5–9 μm. The reported size of roughly 20 × 15 μm fits well with Giardia’s elongated, bipartite body, and on a trichrome stain you would look for the distinctive features of Giardia such as two nuclei giving a doubled, “double-eye” appearance and a ventral adhesive disc. This combination of a pear-shaped trophozoite with a size in that range is most characteristic of Giardia, making it the best match.

Chilomastix mesnili tends to be smaller and more slender, Trichomonas hominis is typically slender and motile with a different morphology, and Dientamoeba fragilis trophozoites are usually smaller and have different nuclear features.