Which of the following organisms is most likely to be the cause of the patient's diarrhea?
A 24-year-old AIDS patient develops chronic abdominal pain, low-grade fever, diarrhea, and malabsorption. Oocysts are demonstrated in the stool.

Isospora belli

The correct answer is D. 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 (no treatment presently available). Diphyllobothrium latum(choice A) is the fish tapeworm and occasionally causes diarrhea. Entamoeba histolytica(choice B) and Giardia lamblia(choice C) are both causes of diarrhea, but they are not specifically associated with AIDS. Microsporidia (choice E) are a protozoan cause of diarrhea, but produce spores rather than oocysts.