Vasovagal syncope (J): This is the most likely diagnosis given the scenario of a young athlete experiencing a brief episode of loss of consciousness immediately after intense physical exertion, with a rapid recovery and no residual symptoms except for shortness of breath. The presence of a systolic murmur that becomes louder with standing could be a red herring, but the clinical context points more towards a vasovagal response to exertion.

Hypertrophic obstructive cardiomyopathy (D): This condition is a common cause of sudden cardiac death in young athletes and can present with syncope, especially after exertion. The presence of a systolic murmur that increases with maneuvers that decrease venous return (such as standing) is consistent with this diagnosis.
Mitral valve prolapse (F): Although less likely to cause syncope directly, mitral valve prolapse can be associated with arrhythmias or significant mitral regurgitation, potentially leading to symptoms. However, it's less directly linked to exertional syncope compared to hypertrophic cardiomyopathy.

Aortic stenosis (A): Although the murmur described does not classically suggest aortic stenosis (which typically radiates to the neck and is louder with squatting), severe aortic stenosis can cause syncope with exertion due to the fixed cardiac output. Missing this diagnosis could be fatal.
Pulmonary embolus (H): Uncommon in a healthy teenager but could occur, especially if there are underlying conditions such as a hypercoagulable state. The shortness of breath could be a clue, but the overall clinical picture is less suggestive.
Seizure (I): Post-ictal confusion might not be present if the seizure was brief, and the shortness of breath could be a manifestation of a seizure. However, there's no clear indication of seizure activity in the provided history.

Carotid sinus hypersensitivity (B): This condition is more common in older adults and would be an unusual cause of syncope in a teenager without other risk factors.
Conversion reaction (C): Now more commonly referred to as a functional neurological symptom disorder, this would be an unusual presentation without psychological stressors or other indicative symptoms.
Hypoglycemia (E): Unlikely without a history of diabetes or other metabolic disorders, and the symptoms do not closely align with hypoglycemic episodes.
Orthostatic hypotension (G): Could be considered if the patient had been dehydrated or had autonomic dysfunction, but the blood pressure and heart rate do not support this diagnosis.
Vertebrobasilar insufficiency (K): Extremely rare in a teenager without significant vascular risk factors or trauma, and the symptoms do not align well with this condition.