A 36-year-old man with AIDS presents with fever, dry cough, and dyspnea. A chest x-ray shows bilateral and diffuse infiltrates. Laboratory studies reveal a low CD4+ cell count. A lung biopsy discloses a chronic interstitial pneumonitis and an intra-alveolar foamy exudate. A silver stain of a bronchoalveolar lavage is shown. Which of the following organisms is the most likely responsible for these pulmonary findings?