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 not being compliant with her regimen of antihypertensive medications During the primary assessment, you find that her blood pressure is significantly higher in her left arm then it is in her right arm What do the signs and symptoms MOST likely indicate?