Question

A 34-year-old woman, G1P0, at 30 weeks gestation is brought to the emergency department due to the sudden onset of severe shortness of breath, orthopnea, and a cough productive of pink, frothy sputum. She recently emigrated from Southeast Asia and has not received routine prenatal care. Her vital signs are temperature 37.0°C (98.6°F), blood pressure 105/65 mm Hg, heart rate 138/min and irregularly irregular, and respiratory rate 32/min. Pulse oximetry shows 87% on room air.

Physical examination reveals a distressed woman using accessory muscles of respiration. Lung auscultation demonstrates diffuse bilateral crackles extending to the apices and prominent expiratory wheezes. Cardiac auscultation reveals a loud $S_1$, a high-frequency sound shortly after $S_2$, and a low-pitched, rumbling mid-diastolic murmur heard best at the cardiac apex.

Laboratory results show:
Hemoglobin: 10.8 g/dL
Leukocyte count: 8,500/mm $^3$

Arterial blood gas on room air:
pH: 7.51
$PaCO_2$: 26 mm Hg
$PaO_2$: 54 mm Hg
$HCO_3^-$: 20 mEq/L

Which of the following is the most important primary pathophysiologic mechanism precipitating this patient's acute respiratory failure?