<prompt> Most common site of aoic dissection is ?Ascending aoaDescending aoaArch of aoaInfrarenal poion of aoa  <response>: Aoic dissection Dissection of the media of the aoa after splitting through intima creating a channel in the media of the vessel wall. Causes- hypeension, marfan;s syndrome, collagen diseases, trauma, weakening of elastic layers of the media due to shear forces. Features:- Commonly seen in thoracic aoaascending aoa. Dissected aoic arch channel gets lined by endothelium, often reopens distally into the aoa causing double barrelled aoa. Atherosclerosis is not a usual cause for dissecting aneurysm. Clinical features- Pain in the chest, back which is excruciating. Features of ischemia. Treatment- Antihypeensives Surgery: using Dacron graft reconstruction of aoa. Reference: SRB;s manual of surgery, 5th edition , page no. 198 