A 46-year-old male with a history of hypertension suddenly experiences severe tearing chest pain radiating to the sternum and epigastrium, accompanied by profuse sweating, persisting for 1 hour without relief. Examination: BP 200/110 mmHg (26.7/14.7 kPa), HR 90 bpm, no abnormal cardiac findings, no abnormal lung findings, ECG shows left ventricular hypertrophy with 0.1 mV ST-segment depression in V4-V6. The most likely diagnosis is: