Answer :USMLE step 2 Mcq 283:A middle aged male patient has chronic diarrhea




Correct Answer: B

Entamoeba histolytica is a large intestine protozoal parasite which causes dysentry. the organisms causes infection in the large intestines and causes the formation of flask shaped ulcers due to the action of a toxin called histolysin. the toxin histolysin causes necrosis of the intetinal mucosa which leads to the formation of ulcers. The unique flask shape of the ulcers is due to the motility of the organism. Salmonella typhi causes infection in the small intestines and not the large intestines. Also its ulcers are longitudnal shaped and not flask shaped. Mycobaterium bovis causes intestinal infection in developing countries where people consume milk without pateurising it. The infection occurs in the small intestine and the ulcers are circumferential in shape. Staphylococcus aureus does not cause intestinal ulceration


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