[ender18d]

Sdiver nailed the easy #1, but I'll try to offer some additional options:

HCM is the most common etiology of sudden death in young athletes, and is consistent with the murmur. Increase index if he's African American. I bet the murmur got quieter when he squatted, or would have if the provider had tried it.

Nurse Tim is also on to something with Marfan's, which may lead to aortic dissection. Marfan's can also cause murmurs through a variety of mechanisms usually related to prolapse of valves or regurgitation.

Abnormalities of the coronary arteries are another possible structural etiology, although they don't explain the murmur quite as nicely.

Myocarditis is a possible infectious cause of sudden cardiac death by way of arrhythmia. Its always worth considering other treatable etiologies.

I'll throw long QT in as another congenital cause of SCD.

Brugada is an interesting thought, although the connection between that and exercise-induced SCD is unclear: usually Brugada kills you in your sleep.

Useful related note for medics in general:
Syncope DURING exercise is much more concerning than syncope AFTER exercise. Just a simple differentiator on whether to get your hackles up. Not saying syncope after exercise is always benign, but if it happens in the middle of exertion, you need to take it very seriously.