Review Question - QID 212380

A 32-year-old man presents to his primary care physician because he has been experiencing intermittent episodes of squeezing chest pain and tightness. He says that the pain is 8/10 in severity, radiates to his left arm, and does not appear to be associated with activity. The episodes started 3 months ago and have been occuring about twice per month. His past medical history is significant for migraines for which he takes sumatriptan. Physical exam reveals no abnormalities and an EKG demonstrates sinus tachycardia with no obvious changes. An angiogram is performed to evaluate coronary artery blood flow. During the angiogram, a norepinephrine challenge is administered and blood flow is observed to decrease initially; however, after 2 minutes blood flow is observed to be increased compared to baseline. Which of the following substances is most likely responsible for the increased blood flow observed at this later time point?