A 30-year old HIV positive patient presents with fever, dyspnoea and non-productive cough. Patient is cyanosed. His chest X-ray reveals bilateral, symmetrical interstitial infiltrates. Which of the following is the most likely diagnosis?

This HIV positive patient is showing clinical features and X-ray findings suggestive of pneumocystis carnii pneumonia. PCP infection occur in HIV positive individuals with CD4 count less than 200. It is an oppounistic fungal pulmonary pathogen that is an impoant cause of pneumonia in the immunocompromised host. Ref: Harrison's Internal Medicine, 18th Edition, Chapter 207