A 24-year-old sex worker from Mumbai develops chronic abdominal pain, low-grade fever, diarrhea, and malabsorption. He saw later found to be positive for HIV. Oocysts are demonstrated in the stool. Which of the following organisms is most likely to be the cause of the patient's diarrhea?

All of the organisms listed are protozoa. There are two intestinal protozoa specifically associated with AIDS that can cause transient diarrhea in immunocompetent individuals but can cause debilitating, and potentially life-threatening chronic diarrhea in AIDS patients. These organisms are Isospora belli, treated with trimethoprim-sulfamethoxazole (or other folate antagonists) and Cryptosporidium parvum. Diphyllobothrium latum is the fish tapeworm and occasionally causes diarrhea. Entamoeba histolytica and Giardia lamblia are both causes of diarrhea, but they are not specifically associated with AIDS.