Entamoeba histolytica infection is initiated when which stage is ingested?

• A. Cyst stage
• B. Trophozoite stage
• C. Spores
• D. Virion

Answer: (A)

Infection with Entamoeba histolytica begins when cysts are ingested. These cysts are the hardy, infectious form that can survive outside the host. After ingestion, cysts pass through the stomach and excyst in the small intestine, releasing trophozoites that colonize the colon and may invade tissue, causing disease. The trophozoite stage is the active form inside the host, not the form taken in from the outside. Spores and virions are not part of this organism’s life cycle.