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?
Pneumocystis carinii
Blastomyces dermatitidis
Mycobacterium a vium
Mycobacterium tuberculosis
Pneumocystis carinii