A 44-year-old woman comes to the physician because of episodic dizziness, double vision and dysarthria She clarifies that these symptoms tend to come on when she raises her right hand above her head such as when brushing her hair or when reaching for something from a shelf. Her neurological examination is normal. Her vital signs are also normal; however, her pulse is stronger at the left radius than the right radius. Furthermore, her blood pressure when measured on the left arm is 145/66 mmHg whereas on her right arm it is 120/58 mmHg Her presentation is due to pathology at which of the following locations?