This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Mycology, Subject name: Microbiology
P. carini causes infection primarily in? 
Select and explain. 
A. Rats, B. Guinea pig, C. Rabbits, D. Humans
A: Rats
Answer: A: Rats 
Explanation: Pneumocystis jirovecii causes infection Primarily in Rats. Pneumocystis jirovecii (previously P. carinii) is the causative organism of Pneumocystis pneumonia paicularly among immunocompromised hosts. P. jiroveci causes PCP in human with HIV if, their CD4+ cells count below 200/uL.