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': "Most common cause of an Abdominal aneurysm is - \nSelect and explain. \nA. Atherosclerosis, B. Trauma, C. Marfan's syndrome, D. Congenital", 'role': 'user'}, {'content': 'Answer: A \nExplanation: Abdominal aortic aneurysm\n\nThe most common aetiology for an abdominal aortic aneurysm is atherosclerosis.\nAtherosclerosis causes arterial wall thinning through medial destruction secondary to a plaque that originates in the intima.\n\nBesides abdominal aorta, atherosclerotic aneurysm also occurs in common iliac arteries, arch and descending part of the thoracic aorta.\n\n\nThe usual site of abdominal aortic aneurism is below the origin of the renal artery and above the bifurcation of the aorta.\n\n\nRemember\n\nMost common cause of an aortic aneurysm (overall) = Atherosclerosis.\nMost common cause of ascending aortic aneurysm = Cystic medial necrosis.\n\nMost common cause of descending aortic aneurysm = Atherosclerosis.\n\n\nMost common cause of aneurysm of aortic arch = Atherosclerosis.\n\n\nA syphilitic aneurysm affects = Ascending aorta.\n\n\nAneurysm of Takayasu arteritis affects = Aortic arch.\n\n\nMost common cause of mycotic abdominal aneurysm = salmonella gastroenteritis.', 'role': 'assistant'}]