Asthma USMLE Step 1 Practice Question
A 43-year-old woman presents with acute-onset pleuritic chest pain and dyspnea 4 days after pelvic surgery. Vital signs reveal HR 118 bpm, RR 28/min, BP 132/84 mmHg, SpO2 88% on room air. Arterial blood gas demonstrates pH 7.48, PaCO2 32 mmHg, PaO2 68 mmHg. CT pulmonary angiography shows wedge-shaped opacity. She denies calf pain. Which physiologic mechanism best explains the ventilation-perfusion mismatch in the affected lung units?
Answer choices
- ALoss of surfactant causing alveolar collapse
- BGlobal hypoventilation with normal A a gradient
- CPerfusion without ventilation causing shunt
- DDecreased diffusion from fibrotic thickening only
- EVentilation without perfusion causing increased dead spaceCorrect answer
- FIncreased intrapulmonary shunting from pulmonary edema due to left heart failure
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