A 62-year-old woman with acute pneumonia affecting the right lower lobe is admitted with fever, cough, and hypoxemia. Chest X-ray confirms consolidation. Arterial blood gas shows PaO₂ 72 mmHg, PaCO₂ 35 mmHg on room air. Alveolar-arterial (A-a) gradient is calculated at 32 mmHg (normal <15). When high-flow oxygen (FiO₂ 0.90) is administered, PaO₂ increases to only 85 mmHg, and the A-a gradient remains elevated at 28 mmHg. Which mechanism best accounts for the persistent hypoxemia despite high supplemental oxygen?