A 31 year old HIV-positive man develops a severe pneumonia. Lower respiratory tract secretions obtained by fiberoptic bronchoscopy with bronchoalveolar lavage and stained with methenamine silver stain demonstrate cup-shaped cysts with sharply outlined walls. Which of the following organisms is the most likely pathogen in this case?

The organism described is Pneumocystis carinii, which is an oppounistic parasite that appears to be more closely related to fungi than to protozoa. Its cyst form, when stained with silver stains, has the distinctive appearance described in the question stem, and is typically found in frothy material that occupies the lumen of alveoli. The trophozoites are smaller and much harder to recognize. Bronchoalveolar lavage is considered much more reliable than induced sputum as a diagnostic specimen. Pneumocystis pneumonia is a common infection among AIDS patients, and is very uncommon in other clinical settings. Formerly, many AIDS patients died with Pneumocystis pneumonia, but the combination of early drug treatment (with trimethoprim/sulfamethoxazole or pentamidine) and prophylaxis (usually with trimethoprim/sulfamethoxazole) has decreased the number of fatal infections. In severe cases, Pneumocystis infection can sometimes be demonstrated in extrapulmonary sites. Candida albicans(choice A) can infect the lung and stain with methenamine silver, but the description of the lavage material would probably include the terms fungal hyphae and yeast forms. Giardia lamblia (choice B) causes diarrhea, rather than pneumonia. Haemophilus influenzae(choice C) and Streptococcus pneumoniae(choice E) are bacteria and would not stain with silver stains. Ref: Levinson W. (2012). Chapter 52. Blood & Tissue Protozoa. In W. Levinson (Ed),Review of Medical Microbiology & Immunology, 12e.