A 52-year-old man with no significant past medical history presents to the emergency department with a 3-day history of productive cough, fever (39.2°C), and progressive dyspnea. On examination, he is tachypneic (RR 28/min), hypoxic (SpO₂ 88% on room air), and has bilateral crackles on auscultation. Chest X-ray shows bilateral infiltrates. ABG reveals: pH 7.32, PaCO₂ 48 mmHg, HCO₃⁻ 22 mEq/L, PaO₂ 62 mmHg on FiO₂ 0.40. He is intubated and placed on mechanical ventilation with PEEP 12 cm H₂O and FiO₂ 0.60, achieving PaO₂ 95 mmHg. Blood cultures grow *Streptococcus pneumoniae*. What is the primary mechanism of hypoxemia in this patient's current condition?