155. A 24-year-old man presents the emergency room after a syncopal episode. He states that before he blacked out he experienced palpitations, and the feeling that his heart was racing. He has no known medical problems and does not smoke, drink, or take any medications or illicit drugs. His ECG is shown in the following figure. Which of the following is the most likely etiology of his symptoms?
a. An accessory AV pathway
b. Rapid impulse formation from the sinoatrial node
c. An ectopic focus in the atria
d. Dual pathways within the AV node
e. Increased vagal influence on the normal pacemaker