A 32-year-old woman with a history of primary amenorrhea and short stature presents to the emergency department with sudden-onset, severe “tearing” chest pain radiating to her back. She is diaphoretic and anxious. Blood pressure is 178/92 mm Hg in the right arm and 138/86 mm Hg in the left arm. Pulses are diminished in the left radial artery compared with the right. A new early diastolic murmur is heard at the left sternal border. Which of the following best describes the underlying lesion responsible for this presentation?