Vascular Disease USMLE Step 1 Practice Question

A 72-year-old man with a 15-year history of hypertension presents to the emergency department with sudden-onset, severe, tearing substernal chest pain radiating to the interscapular region. He reports the pain began abruptly while he was walking. His current medications include lisinopril and amlodipine. Vital signs on presentation: BP 188/115 mmHg in the right arm and 152/98 mmHg in the left arm, HR 103/min, RR 24/min, temperature 37.1°C, SpO2 99% on room air. Physical examination reveals a new early diastolic murmur. Chest X-ray demonstrates a widened mediastinum and possible left pleural effusion. A 12-lead ECG shows normal sinus rhythm with no acute ST-segment changes. Which of the following is the most appropriate next step in diagnosis?