Which organism has a life cycle that involves ingestion of cysts and development of trophozoites in the small intestine?

• A. Giardia Intestinalis Cyst
• B. Giardia Intestinalis Trophozoite
• C. Plasmodium species
• D. Trichomonas vaginalis trophozoite

Answer: (B)

Giardia intestinalis uses a cyst as the infectious form. When those cysts are swallowed, they excyst in the proximal small intestine and release trophozoites that then multiply and attach to the intestinal lining. Some trophozoites pass through and form cysts again that are shed in stool. This sequence—ingestion of cysts followed by development of trophozoites in the small intestine—is the hallmark Giardia life cycle, so the trophozoite form is the best match among the choices. Other organisms either lack a cyst stage or don’t develop trophozoites in the small intestine.