This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Misc., Subject name: Microbiology
Most common cause of pneumonia in AIDS patients 
Select and explain. 
A. Pneumococci, B. Staphylococci, C. Pneumocystis carni, D. All of the above
C: Pneumocystis carni
Answer: C: Pneumocystis carni 
Explanation: (C) Pneumocystis carnii > Pneumocystis carini is most common cause of pneumonia in AIDS.