A 50-year-old man consults you because he has been having transient periods of rapid heart beat accompanied by sweating, flushing, and a sense of impending doom. Examination is unrevealing, with no evidence of arrhythmia at the time of the exam. However, the man's wife is a nurse, so the physician asks that she take vital signs the next time one of the episodes occurs. She does, and demonstrates a BP of 195/140 with heart rate 160/min during the episode. She promptly brings him to you, but the spell is over by the time that he is seen. Urinary measurement of which of the following would most likely be diagnostic in this case?