A trichrome-stained fecal smear shows an organism approximately 11 × 9 μm. What is the identification?

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Multiple Choice

A trichrome-stained fecal smear shows an organism approximately 11 × 9 μm. What is the identification?

Explanation:
A key idea here is that cyst-forming intestinal protozoa have characteristic size ranges that help distinguish them on staining. Giardia duodenalis cysts are typically oval to ellipsoidal and measure about 8–12 μm in one dimension and 7–10 μm in the other. Seeing an organism around 11 × 9 μm on a trichrome-stained fecal smear fits this size range very well, making Giardia cyst the most likely identification. Other organisms listed are less consistent with these measurements. Endolimax nana cysts are smaller, usually around 6–8 μm. Entamoeba hartmanni cysts also fall in a smaller size range and are generally identified by their own cyst morphology. Dientamoeba fragilis is better known for its trophozoite form in stool rather than a cyst, so a cyst of this size is less characteristic for that organism. Thus, the observed size and typical cyst morphology on trichrome stain align best with Giardia duodenalis cyst.

A key idea here is that cyst-forming intestinal protozoa have characteristic size ranges that help distinguish them on staining. Giardia duodenalis cysts are typically oval to ellipsoidal and measure about 8–12 μm in one dimension and 7–10 μm in the other. Seeing an organism around 11 × 9 μm on a trichrome-stained fecal smear fits this size range very well, making Giardia cyst the most likely identification.

Other organisms listed are less consistent with these measurements. Endolimax nana cysts are smaller, usually around 6–8 μm. Entamoeba hartmanni cysts also fall in a smaller size range and are generally identified by their own cyst morphology. Dientamoeba fragilis is better known for its trophozoite form in stool rather than a cyst, so a cyst of this size is less characteristic for that organism.

Thus, the observed size and typical cyst morphology on trichrome stain align best with Giardia duodenalis cyst.

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