A 58-year-old male presents with the sudden onset of excruciating pain. He describes the pain as beginning in the anterior chest, radiating to the back, and then moving downward into the abdomen. His blood pressure is found to be 160/110. Your differential diagnosis includes myocardial infarction; however, no changes are seen on ECG, and you consider this to be less of a possibility. You obtain an x-ray of this patient's abdomen and discover a "double-barrel" aoa. This abnormality most likely results from