A 29-year-old woman is evaluated for shortness of breath while mountain climbing. She is currently at an elevation of 3200 meters (10,500 feet) and has developed cough and dyspnea, which are present both with exertion and at rest; these symptoms prevent her from continuing the climb. Her medical history is unremarkable, and she takes no medications.

On physical examination, temperature is 36.0 °C (96.8 °F), blood pressure is 154/90 mm Hg, pulse rate is 120/min, and respiration rate is 22/min; BMI is 23. Oxygen saturation is 86% breathing ambient air. She is in moderate respiratory distress. Pulmonary examination reveals a few bibasilar crackles.

Supplemental oxygen is given and arrangements are made for descent from altitude.

Which of the following is the most appropriate adjunctive treatment for this patient?