A 54-year-old woman with a history of hypertension and obesity presents with acute onset pleuritic chest pain and shortness of breath. She recently completed a 10-hour flight. Vitals: T 98.6 F, HR 112, BP 128/76, RR 24, SpO2 92% on room air. Her Wells score for pulmonary embolism is calculated as 6 (high probability). What is the most appropriate next diagnostic step?