A 28-year-old HIV patient presents with non-productive cough and difficulty in breathing which was progressively increasing. Chest X-ray shows diffuse bilateral interstitial infiltrates along with pneumatocele. Gomori's methenamine silver staining of the bronchoalveolar lavage shows the following. The most probable causative organism is

The given clinical picture and the GMS stain showing the black colored stained cysts resembling the crushed ping pong balls are suggestive of pneumocystis pneumonia. It is caused by Pneumocystis jirovecii. Patients with PCP generally present with fever and cough that is usually non-productive. Finding on chest x-ray is either a normal film if the disease is suspected early, or diffuse bilateral interstitial infiltrates. The classic finding of dense perihilar infiltrate is unusual in patients with AIDS. Treatment: Trimethoprim-sulfamethoxazole and pentamidine isethionate. Reference: Ananthanarayan and Paniker&;s Textbook of Microbiology Tenth edition