An 66--year--old AIDS patient with 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?
Blastomyces dermatitidis Mycobacterium a vium Mycobacterium tuberculosis Pneumocystis carinii