ECG of the Month: 21-Year-Old Male Presents with Exertional Syncope with an Abnormal ECG

A 21-year-old man with no significant past medical history is admitted to the hospital for evaluation of recurrent syncope of unknown etiology.

The day of admission, he felt palpitations and dyspnea while arguing with his significant other. Subsequently, he noticed visual blurring and lowered himself to the floor. The patient was uncertain of the sequence of events that immediately followed. His significant other, who witnessed the event, called emergency medical services.

He reports a history of palpitations during periods of emotional stress and two prior episodes of syncope while playing basketball. After both episodes of syncope, his only recollection is regaining consciousness while people were attending to him. He has never had any cardiovascular testing prior. His family history is remarkable for a brother diagnosed with possible hypertrophic cardiomyopathy, who received an implantable cardioverter-defibrillator at the age of 30 years.

Laboratory values are unremarkable, although a troponin I measurement is 0.58 ng/mL (normal <0.11 ng/mL). The patient's electrocardiogram (ECG) is displayed in Figure 1.

Figure 1

Figure 1

Which of the following is most likely responsible for his recurrent syncopal episodes?

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