A 34-year-old woman presents with sudden-onset pleuritic chest pain and dyspnoea. She uses oestrogen-containing contraception and returned from a long-haul flight 3 days ago. Vitals: BP 118/72 mm Hg, HR 104, RR 22, SpO₂ 96% on room air. Her Wells score is low. According to evidence-based PE diagnostic algorithms, what is the best next step?