A 28-year-old man presents to the Emergency Department with sudden onset severe shortness of breath and pleuritic chest pain. He has no significant past medical history but returned from a long-haul flight 3 days ago. Observations: HR 110 bpm, RR 24/min, BP 120/80 mmHg, SpO2 93% on air. His Wells score is calculated as 5. What is the most appropriate next diagnostic step?