A 48-year-old woman presents with sudden-onset pleuritic chest pain and dyspnoea. She returned from a 10-hour flight 3 days ago. Her heart rate is 110 beats/min, blood pressure 118/74 mm Hg, respiratory rate 24 breaths/min and oxygen saturation 95% on room air. She has no signs of DVT on examination. Using the Wells score, she is assessed as having an intermediate pre-test probability of pulmonary embolism. According to current diagnostic strategies, what is the most appropriate next investigation?