A 32-year-old male trekker from Delhi ascends to 4500 m altitude in the Himalayas over 2 days. On day 3, he develops severe headache, nausea, and dyspnea at rest. His respiratory rate is 28/min, oxygen saturation by pulse oximetry is 78%, and he is confused. Arterial blood gas shows pH 7.32, PaCO₂ 28 mmHg, HCO₃⁻ 14 mEq/L, and PaO₂ 42 mmHg. Which of the following is the primary mechanism of hypoxemia in this patient?