Which of the following is the largest intestinal protozoa?

Balantidium coli is the largest of the human intestinal protozoa. It can produce a spectrum of large-intestinal disease called as balantidiasis. Infective cysts can be transmitted from person to person and through water, but many cases are due to the ingestion of cysts derived from porcine feces. Many patients remain asymptomatic. In symptomatic individuals, the pathology in the bowel--both gross and microscopic--is similar to that seen in amebiasis. Balantidiasis, unlike amebiasis, does not spread hematogenously to other organs. The diagnosis is made by detection of the trophozoite stage in stool or sampled colonic tissue. Tetracycline (500 mg four times daily for 10 days) is an effective therapeutic agent. Ref: Weller P.F. (2012). Chapter 215. Protozoal Intestinal Infections and Trichomoniasis. In D.L. Longo, A.S. Fauci, D.L. Kasper, S.L. Hauser, J.L. Jameson, J. Loscalzo (Eds), Harrison's Principles of Internal Medicine, 18e.