A fib: “You can’t Handle the Truth”

Sean fancied himself a remarkable skier. Every December, after buying his Dogs of the DOW with which he has a track record of significantly outperforming his working stiff “friends” (faculty in the basic sciences), he would fly to Steamboat, CO to ski on their famous Champagne powder. There was also plenty of “toasting” during his visits, beginning with breakfast, a custom he practiced and perfected over years. Sean chose to ignore all of the friendly warnings from his colleagues who suggested that his infrequent bouts of shortness of breath after minimal exertion might perhaps be an indication that something was amiss in his chest.  He “could not handle the truth”. Then his symptoms could no longer be ignored:  on his most recent trip, after a breakfast of aged Colorado bourbon, he took the lift to the peak where he felt a pounding in his chest, shortness of breath and light-headedness. He could not continue and needed the ski patrol to remove him from the mountain in a sled before transfer to an ambulance then to a clinic. An EKG revealed clear evidence of severe atrial fibrillation so he was airlifted to another nearby hospital where he underwent ablation of his atrioventricular node. He was also implanted with a pacemaker. In the image, what is the most likely location of the atrioventricular node?

The atrioventricular node is best found nearest which numbered line?