A 58-year-old man with a 20-year history of poorly controlled hypertension presents to the emergency department with sudden-onset, severe “tearing” chest pain that began 30 minutes ago while watching TV. The pain radiates to his back between the scapulae. He is anxious and diaphoretic. BP is 188/96 mm Hg in the right arm and 162/88 mm Hg in the left arm; HR is 112/min; RR is 22/min; O2 sat is 97% on room air. On exam, pulses are diminished in the left radial artery compared with the right. Cardiac auscultation reveals a new, high-pitched, blowing decrescendo diastolic murmur along the left sternal border. A portable chest X-ray shows widening of the mediastinum. The patient is hemodynamically stable and has normal renal function.

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