Q1: Answer: C
Cystoisospora belli (formerly known as Isospora belli) is a gastrointestinal protozoan. In patients with AIDS and other immunodeficiencies, it is an opportunistic pathogen that can cause watery diarrhea and weight loss. Infections are acquired by the ingestion of sporulated oocysts from food or water contaminated with human feces. In general, protozoal infections do not cause peripheral or tissue eosinophilia; however, Cystoisospora infection is an exception to this rule.
Diarrhea and peripheral eosinophilia in an immunocompromised individual should raise concern for Cystoisospora infection. The other protozoal infections listed can cause diarrhea, but do not cause eosinophilia.
Reference
1. Goodgame, R.W. Understanding intestinal spore-forming protozoa: Cryptosporidia, microsporidia, isospora, and cyclospora. Ann Intern Med. 1996;124:429-41.