A 42-year-old man presents with sudden onset severe, tearing central chest pain radiating through to his back. He is hypertensive (BP 190/110 mmHg in right arm, 160/95 mmHg in left arm) and tachycardic (110 bpm). An ECG shows sinus tachycardia with non-specific ST depression laterally. Chest X-ray shows a widened mediastinum. What is the most likely diagnosis?