A 69-year-old man comes to the emergency department with sudden-onset severe lower abdominal and back pain that began 30 minutes ago while he was watching TV. He feels lightheaded and diaphoretic. He has a 50-pack-year smoking history and long-standing hypertension. On examination, he is pale; temperature is 36.8°C (98.2°F), blood pressure is 78/46 mm Hg, pulse is 128/min, and respirations are 24/min. His abdomen is diffusely tender, and a pulsatile midline mass is palpated. Which of the following best describes the typical anatomic location of the underlying lesion responsible for this patient’s presentation?