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: Radiology
A patient suffering from AIDS presents with history of dyspnea and non- productive cough x-ray shows bilateral perihilar opacities without pleural effusion and lymphaden-opathy. Most probable etiological agent is : 
Select and explain. 
A. Tuberculosis, B. CMV, C. Kaposis sarcoma, D. Pneumocystis carinii
D: Pneumocystis carinii
Answer: D: Pneumocystis carinii 
Explanation: D i.e. Pneumocystic carinii