A 56-year-old man is undergoing chemotherapy for leukemia. He has developed fever, nonproductive cough, dyspnea, pleuritic chest pain, and hemoptysis over the past week. A chest CT scan shows multiple 1- to 4-cm nodular densities having surrounding areas of ground-glass infiltrate (halo sign). Bronchoalveolar lavage is performed, and microscopic examination of the fluid shows narrow branching septate hyphae. A CBC shows Hgb, 13 g/dL; Hct, 38.7%; WBC count, 2000/mL; and platelet count, 200,100/mL. He has most likely developed an infection with which of the following organisms?