A 58-year-old man with a 3-day history of acute dyspnea, pleuritic chest pain, and tachycardia (HR 110/min, RR 28/min) is admitted to the ward. His D-dimer is elevated at 2.5 µg/mL. Chest X-ray shows a wedge-shaped opacity in the right lower lobe. He is haemodynamically stable with normal blood pressure (128/82 mmHg) and oxygen saturation 94% on room air. What is the most appropriate next step in management?