A 50-year-old woman presents to the emergency department with pleuritic chest pain and dyspnoea. She has had recent lower limb surgery and is using the oral contraceptive pill. Her Well's score for pulmonary embolism is 5.5 (moderate probability). Vital signs: BP 126/80 mmHg, HR 104 bpm, RR 24/min, SpO2 95% on room air. What is the most appropriate next investigation?