A 68-yeаr-оld mаn wаs brоught tо the Emergency Department by ambulance with sudden onset of severe chest pain, described as tearing, that radiates down the back for the past 25 minutes. He admits to a long-standing history of hypertension that has not been well controlled for the past 25 years. He states that he feels short of breath, is beginning to feel dizzy, and his legs are becoming weak. His vital signs are as follows: RR 24/min, HR 110/min, BP 208/152 mm Hg (right arm) and 212/156 mm Hg (left arm). He is afebrile. Which of the following conditions is the most likely diagnosis for this patient?