<prompt> 24 yrs old AIDS patient develops chronic abdominal pain, low grade fever, diarrhea, and malabsorption. Oocyst demonstrated in stool. Likely cause of his diarrhea is -E. histolyticaGiardiaTrichomonasIsospora belli  <response>: Option 4 The most common oppounistic Protozoa infections in untreated AIDS patients include the following: Toxoplasma gondii, Isospora belli, Cryptosporidium species. Clinical feature of Isospora belli: fever, abdomen pain, watery nonbloody diarrhea. Disease is more severe in HIV patients resembling cryptosporidiosis. Wet mount or modified Z.N. staining 0.1 - 1 H2SO4 can be done for demonstration of oocysts of Isospora belli. Image: Isospora belli: 23-36 mm in size, oval, uniformly acid fast 1 H2SO4, contains two sporocyst each bearing four sporozoites. option 1 ,2 Oocyst are not seen in case of E. histolytica and Giardia option 3 Trichomonas cause vaginal infection 