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:

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.