A 34-year-old man with acute myeloid leukemia undergoing induction chemotherapy presents with fever, productive cough, and pleuritic chest pain. High-resolution CT chest shows a nodule with surrounding ground-glass opacification (halo sign). Bronchoscopic alveolar lavage is performed, and histopathology reveals fungal hyphae. The structure marked **B** in the diagram—the acute-angle (45°) dichotomous branching pattern—is the key distinguishing feature observed. Which of the following organisms is most likely responsible for this patient's infection?