This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: JIPMER 2019, Subject name: Microbiology
A retroviral positive patient presents with frequent diarrheal episodes. Stool examination reveals acid-fast oocyst of size 4-6 microns. What could be the probable diagnosis? 
Select and explain. 
A. Toxoplasma gondii, B. Cryptosporidium parvum, C. Coccidioides immitis, D. Sarcocystis
B: Cryptosporidium parvum
Answer: B: Cryptosporidium parvum 
Explanation: In HIV positive patients, following organisms cause diarrhea. Cryptosporidium parvum- 4-6 micron oocyst Cyclospora - 2-12 micron oocyst Isospora belli23-36- micron oocyst