Section: USMLE

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

1. Diphyllobothrium latum
2. Entamoeba histolytica
3. Giardia lamblia
4. Isospora belli
5. Microsporidia

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 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.

Microsporidia are a protozoan cause of diarrhea but produce spores rather than oocysts.