This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Pathology
[{'content': 'Which is the most common cause for dissecting aneurysm of thoracic aoa? \nSelect and explain. \nA. Atherosclerosis, B. Medial degeneration, C. Syphilis, D. Trauma', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Aoic dissection is a catastrophic illness characterized by dissection of blood in between and along the laminar planes of the media,with the formation of a blood filled channel within the aoic wall; this channel often ruptures through the adventitia and into various spaces, where it causes either massive hemorrhage or cardiac tamponade.Hypeension is the major risk factor for aoic dissection.Ref: Robbins Basic Pathology, 8th Ed, page 359-360', 'role': 'assistant'}]