Dehydration/Hypovolemia: This is the most likely diagnosis, as intense physical activity can lead to significant fluid loss through sweating, potentially causing a drop in blood pressure and resulting in syncope (passing out).

Vasovagal Syncope: A common cause of fainting, often triggered by stress, pain, or prolonged standing, which can be exacerbated by exercise.
Post-Exertional Hypotension: A condition where blood pressure drops after exercise due to vasodilation, leading to reduced blood flow to the brain and potential fainting.
Cardiac Arrhythmias: Certain arrhythmias, such as supraventricular tachycardia (SVT) or ventricular tachycardia (VT), can be triggered by exercise and lead to syncope.

Aortic Dissection: Although rare, this is a life-threatening condition that can be precipitated by intense physical exertion, especially in individuals with pre-existing hypertension or aortic aneurysm.
Pulmonary Embolism: Exercise can increase the risk of pulmonary embolism, especially in individuals with underlying risk factors such as deep vein thrombosis or recent surgery.
Cardiac Structural Abnormalities: Conditions like hypertrophic cardiomyopathy or coronary artery anomalies can lead to exercise-induced syncope and are critical to diagnose due to their potential for sudden cardiac death.

Long QT Syndrome: A genetic disorder affecting the heart's electrical system, which can lead to life-threatening arrhythmias during exercise.
Catecholaminergic Polymorphic Ventricular Tachycardia (CPVT): A rare genetic disorder that can cause arrhythmias and syncope during physical activity or emotional stress.
Mitral Valve Prolapse: Although often asymptomatic, in rare cases, mitral valve prolapse can lead to arrhythmias or outflow tract obstruction, potentially causing syncope during exercise.