A 62-year-old man with a history of long-standing uncontrolled hypertension presents to the emergency department with sudden onset of severe “tearing” chest pain that radiates to the back. He is anxious and diaphoretic. Blood pressure is 190/110 mm Hg in the right arm and 160/95 mm Hg in the left arm. Radial pulses are asymmetric. Cardiac auscultation reveals a new decrescendo diastolic murmur best heard along the left sternal border. ECG shows nonspecific ST-T changes. Chest x-ray demonstrates a widened mediastinum. He is hemodynamically stable and has normal renal function.

Which of the following is the most appropriate next diagnostic study to confirm the suspected diagnosis?