A 65-year-old man presents with a 2-week history of abdominal discomfort. Physical examination reveals a pulsatile, abdominal mass in the periumbilical region. A CT scan shows a segment of abdominal aorta proximal to the bifurcation that is dilated (5 cm) and calcifi ed. The patient is scheduled for corrective surgery but suffers a massive stroke and expires. The abdominal aorta is examined at autopsy (shown in the image). Which of the following is the most likely underlying cause of this patient’s abdominal mass?