3. The correct answer is D. All 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 currently 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.