A 66-year-old woman presents with a stabbing pain in the middle of her chest that radiates to her back. She tells you that the pain suddenly began about 30 minutes ago and has been severe since the onset. She has a history of hypertension, but admits to being noncompliant with her antihypertensive medications. When you assess her, you find that her blood pressure is significantly higher in her left arm than it is in her right arm. What do these signs and symptoms MOST likely indicate