A normally healthy 9-year-old boy presents with a fever, cough, and lower respiratory symptoms (no upper respiratory tract symptoms). He has been ill for 4 days. His chest sounds are consistent with pneumonia, so a chest radiograph is obtained. The radiograph shows small, patchy infiltrates with hilar adenopathy. His blood smear shows small, nondescript yeast forms inside monocytic cells. What is the most likely causative agent?