<prompt> 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 dermatitidisMycobacterium a viumMycobacterium tuberculosisPneumocystis carinii  <response>: Pneumocystis carinii 