High Altitude and Hypoxia

A 28-year-old female mountaineer from Mumbai acclimatizes at 3500 m for 5 days and then ascends to 5500 m. Within 12 hours, she develops severe dyspnea, orthopnea, and pink frothy sputum. Chest auscultation reveals bilateral crackles. Pulse oximetry shows SpO₂ 65%. Chest X-ray shows bilateral alveolar infiltrates with a normal cardiac silhouette. Which of the following best explains the pathophysiology of her acute condition?

At high altitude, the primary stimulus for increased ventilation in the first few hours is mediated by which of the following?

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?