Cysts are the infective stage of which intestinal flagellate?

• A. Balantidium coli
• B. Dientamoeba fragilis
• C. Entamoeba coli
• D. Giardia lamblia

Answer: (D)

The key idea is that the infectious form for this intestinal parasite is a cyst that survives in the environment and is ingested to start infection. Giardia lamblia is a flagellated protozoan whose life cycle relies on hardy cysts as the primary infectious stage. When these cysts are swallowed, they resist stomach acid and excyst in the small intestine to release trophozoites that attach to the intestinal lining and cause illness. This cyst-first transmission is a hallmark of Giardia and is why it’s the correct choice here. Balantidium coli is a ciliate, not a flagellate, and its infectious form is not the same cyst form described for Giardia. Dientamoeba fragilis is a flagellate but does not have a clearly established cyst phase as the infectious form, and Entamoeba coli is an amoeba, not a flagellate, with a cyst stage that is not the one being asked about.