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: Medicine
Chest X-ray revealed bilateral, symmetrical interstitial infiltrates in a 30-year old HIV positive patient who presented with fever, dyspnoea, non-productive cough, and cyanosis. The MOST likely diagnosis is: 
Select and explain. 
A. Tuberculosis, B. Cryptococcosis, C. Pneunocystis carinii pneumonia, D. Toxoplasmosis
C: Pneunocystis carinii pneumonia
Answer: C: Pneunocystis carinii pneumonia 
Explanation: Chest X-ray findings of bilateral, symmetrical interstitial infiltrates in a HIV positive patient with respiratory symptom is almost diagnostic of Pneunocystis carinii pneumonia. Ref: Harrisons principles of internal medicine 18th edition, chapter 189.