A 46-year-old woman presents with pleuritic chest pain and dyspnoea. She recently had a knee replacement 2 weeks ago. Vital signs: heart rate 112 beats/min, respiratory rate 24 breaths/min, blood pressure 118/74 mmHg, oxygen saturation 94% on room air and temperature 37.2 °C. Examination is otherwise unremarkable. Her Wells score for pulmonary embolism is calculated as moderate probability. What is the most appropriate next investigation?