This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: AIIMS 2018, Subject name: Microbiology
A known HIV patient on anti - retroviral therapy presented with diarrhea of six months duration. Stoll microscopy was done in which 10-30 micrometer cysts were seen, Kinyoun stain was positive. What is the most likely diagnosis? 
Select and explain. 
A. Cystoisospora, B. Cryptosporidium, C. Balantidium coli, D. Strongyloides
A: Cystoisospora
Answer: A: Cystoisospora 
Explanation: The identification of the parasite is based on the size of oocyst since the size of oocyst is 20 to 30 Micron the most likely parasite is Isospora. If the size was 4-8 micron the parasite would have been Cryptosporidium parvum.