A 41-year-old woman presents to the emergency department with pleuritic chest pain and shortness of breath. She recently had knee surgery and has been relatively immobile. Her heart rate is 110 beats/min, respiratory rate 24 breaths/min, and oxygen saturation 95% on room air. She has no leg swelling. Using the Wells score, she is judged to be at moderate pre-test probability for pulmonary embolism. What is the most appropriate next diagnostic step?