This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Microbiology
24 Year old AIDS pt. develops chronic abdominal pain, low grade fever, diarrhoea, malabsorption. Oocysts demonstrated in stool. Likely cause of his diarrhoea is - 
Select and explain. 
A. E. histolytica, B. G lamblia, C. Microsporidia, D. Isospora belli
D: Isospora belli
Answer: D: Isospora belli 
Explanation: None