A 58-year-old man presents with sudden onset pleuritic chest pain and dyspnoea. He returned from a long-haul flight two days ago. Heart rate is 110 beats/min, blood pressure 130/78 mmHg, respiratory rate 24 breaths/min and oxygen saturation 95% on room air. Examination is otherwise unremarkable. His Wells score for pulmonary embolism is calculated as 5.5 (moderate probability). What is the most appropriate next diagnostic step?