A 32-year-old man with newly diagnosed HIV (CD4 count 85 cells/μL) presents with progressive dyspnea, fever, and a dry cough for 2 weeks. Chest X-ray shows bilateral perihilar ground-glass infiltrates. Induced sputum with GMS staining confirms Pneumocystis jirovecii. The drug marked **D** in the diagram is initiated at a dose of 15–20 mg/kg/day in divided doses for 21 days. Which of the following best describes the mechanism of action of this drug combination in treating PCP?