## Distinguishing E. histolytica from E. moshkovskii ### Key Discriminating Feature **Key Point:** The ingestion of red blood cells (erythrophagocytosis) is the single most reliable feature that distinguishes E. histolytica trophozoites from the morphologically similar E. moshkovskii. ### Comparative Table | Feature | E. histolytica | E. moshkovskii | | --- | --- | --- | | **RBC ingestion** | **Present** (pathognomonic) | Absent | | Nucleus | Single, central karyosome | Single, central karyosome | | Size | 12–15 μm (range 10–20 μm) | 8–10 μm | | Motility | Progressive, pseudopodia | Progressive, pseudopodia | | Pathogenicity | Invasive, causes dysentery | Non-pathogenic, commensal | | Habitat | Colon (invasive) | Colon (non-invasive) | ### Why This Matters **Clinical Pearl:** E. moshkovskii is a free-living, non-pathogenic amoeba that can colonize the human intestine. Without the RBC-ingestion criterion, morphology alone cannot reliably separate the two species in stool smears. **High-Yield:** When you see RBCs within the cytoplasm of an amoeba trophozoite on stool microscopy, it is E. histolytica until proven otherwise. This is the gold-standard morphological discriminator in resource-limited settings. ### Additional Discriminators (Secondary) - **Isoenzyme electrophoresis:** E. histolytica has a characteristic pattern; E. moshkovskii differs. - **Culture characteristics:** E. histolytica grows on special media; E. moshkovskii is easier to culture in vitro. - **Molecular methods (PCR):** Definitive but not routine in field microscopy. **Mnemonic:** **RBC-HIS** — Red Blood Cell ingestion = Histolytica (pathogenic).
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