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You are seeing a 14 year old boy who presents with shortness of breath and chest pain while playing basketball. He has noticed it for the last few weeks, whenever he plays basketball. He has no significant past medical history, and he has not had a syncopal event. On exam, he has a 2/6 harsh systolic murmur best heard at the left lower sternal border. The murmur is louder with the Valsalva maneuver and softer when the patient goes from standing to squatting.

What ECG finding will you be looking for closely?