A 32-year-old woman presents to the emergency department with sudden-onset pleuritic chest pain and shortness of breath after a long-haul flight from London. She takes the combined oral contraceptive pill. Vital signs: temperature 37.2 °C, blood pressure 118/76 mmHg, heart rate 112 bpm, respiratory rate 24 breaths/min, SpO2 93% on room air. Examination is otherwise unremarkable. Her Wells score for pulmonary embolism is 6. What is the most appropriate next investigation?