A 27-year-old previously healthy woman presents with sudden-onset pleuritic chest pain and shortness of breath. She returned from a long-haul flight 2 days ago and takes combined oral contraceptives. Vital signs: temperature 98.6 F, blood pressure 118/70 mm Hg, heart rate 110 per minute, respiratory rate 24 per minute, oxygen saturation 94 percent on room air. Wells score is intermediate. What is the most appropriate next diagnostic step?