A 52-year-old woman presents with acute onset pleuritic chest pain and dyspnoea. She returned from a long-haul flight 3 days ago. Her heart rate is 115 beats per minute, respiratory rate 24 breaths per minute, oxygen saturation 94 percent on room air and blood pressure 118/72 mmHg. There are no clinical signs of DVT. Her Wells score is calculated as 4.5 (moderate probability). What is the most appropriate next diagnostic step?