An AIDS patient with clinical pneumonia has a bronchoalveolar lavage that demonstrates small, "hat- shaped" structures in alveoli that are about the size of an erythrocyte and stain with silver stains. The microorganism involved is most likely which of the following?

This is the classic appearance of Pneumocystis cysts. Pneumocystis is a common (and dangerous) cause of pneumonia in AIDS patients . The diagnosis is now frequently made by bronchoalveolar lavage, which is much more effective at demonstrating the organism than is either sputum or blood culture. Aspergillus fumigatus is a typical fungus, and hyphae would probably have been seen in the lavage material. Blastomycosis is caused by a dimorphic fungus that grows in mammalian tissues as a round, multinucleate, budding cell 8-15 micrometers in diameter. Mycobacteria are small, acid-fast rods. Ref: Ray C.G., Ryan K.J. (2010). Chapter 45. Candida, Aspergillus, Pneumocystis,and Other Oppounistic Fungi. In C.G. Ray, K.J. Ryan (Eds), Sherris Medical Microbiology, 5e.