A 58-year-old man with a history of recent orthopedic surgery (tibial fracture fixation 5 days ago) presents to the emergency department with acute onset dyspnea, pleuritic chest pain, and tachycardia. Vital signs: BP 128/82 mmHg, HR 112/min, RR 28/min, SpO₂ 88% on room air. Physical examination reveals unilateral calf swelling and tenderness. Chest X-ray is normal. ECG shows sinus tachycardia with T-wave inversion in leads V1–V3. D-dimer is elevated at 2.8 μg/mL. What is the most appropriate next diagnostic step?