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?: (A) Aspergillus fumigatus, (B) Blastomyces dermatitidis, (C) Mycobacterium avium, (D) Pneumocystis carinii
Answer is D. 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.