A 28-year-old previously healthy man comes to the urgent care clinic due to shortness of breath. He is on a ski vacation in Breckenridge, Colorado, situated at an altitude of 2926 m (9600 ft). On arrival 4 days ago, he experienced mild headache and nausea that resolved spontaneously, but since yesterday he has had worsening dyspnea and cough. He now feels short of breath even with minimal exertion. Temperature is 37.3 C (99.1 F), blood pressure is 134/82 mm Hg, pulse is 98/min, and respirations are 22/min. Oxygen saturation is 86% on ambient air, which rapidly improves to 95% with supplemental oxygen. Physical examination shows pink mucous membranes, flat neck veins, bilateral inspiratory crackles, and no heart murmurs or pedal edema. Chest x-ray reveals patchy alveolar infiltrates. Which of the following is the most likely cause of this patient's current condition?
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