Ans. is 'c' i.e., Atherosclerosis Aneurysmo Aneurysm is a localized abnormal dilatation of blood vessel or the wall of hearto When an aneurysm is bounded by true arterial wall, it is called a true aneurysm.o When wall, is ruptured and there is collection of blood outside the vessel wall {extravaseular hematoma) that is freely communicating with vessel wall - false aneurysm. Most common cause of false aneurysm is post myocardial infarction rupture.o Aneurysm due to infection that causes weakening of vessel wall - mycotic aneurysm.o The two most important causes of aortic aneurysm (not only abdominal part but in any part of aorta) are atherosclerosis and cystic medial degeneration of arterial media.Abdominal aortic aneurvsmo The most common etiology for abdominal aortic aneurysm is atherosclerosis.o Atherosclerosis causes arterial wall thining through medial destruction secondary to plaque that originates in the intima.o Besides abdominal aorta, atherosclerotic aneurysm also occur in common iliac arteries, arch and descending part of thoracic aorta.o The usual site of abdominal aortic aneurism is below the origin of renal artery and above the bifurcation of the aorta.Remembero Most common cause of aortic aneurysm (overall) = Atherosclerosis.o Most common cause of ascending aortic aneurysm = Cystic medial necrosis.o Most common cause of descending aortic aneurysm = Atherosclerosis.o Most common cause of aneurysm of aortic arch = Atherosclerosis.o Syphilitic aneurysm affect = Ascending aorta, a Aneurysm ofTakayasu arteritis affects = Aortic arch.o Most common cause of mycotic abdominal aneurysm = salmonella gastroenteritis.