A 62-year-old woman presents with sudden-onset pleuritic chest pain and dyspnoea that started 2 hours ago. She returned from a long-haul flight 3 days ago. Heart rate is 112 beats/min, blood pressure 128/78 mmHg, respiratory rate 24 breaths/min and oxygen saturation 92% on room air. There is mild calf tenderness and swelling on the left. Her Wells score for pulmonary embolism is calculated as high probability. What is the most appropriate initial diagnostic investigation?