A 58-year-old woman is evaluated on post-operative day 3 following a total hip replacement. She reports sudden onset of sharp right-sided chest pain that worsens with deep breathing and severe dyspnea beginning 2 hours ago. She has been immobilized in bed since surgery. Vital signs: HR 118 bpm, RR 24 breaths/min, BP 114/72 mmHg, SpO2 88% on room air. Lung auscultation reveals clear, equal breath sounds bilaterally. Arterial blood gas: PaO2 57 mmHg, PaCO2 31 mmHg, pH 7.49. Chest X-ray shows no consolidation, pleural effusion, or infiltrate. CT pulmonary angiography demonstrates a filling defect in the right lower lobe pulmonary artery; the affected lung parenchyma appears normal on imaging.
Which of the following best explains the mechanism of this patient's hypoxemia?