Which is the most common cause for dissecting aneurysm of thoracic aoa?

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