Stroke: Although stroke is less common in young adults, it is a critical consideration, especially with focal neurological deficits like right-sided weakness. The combination of syncope and unilateral weakness could indicate a stroke, particularly if the syncope was due to a cardiac cause that also led to an embolic stroke.

Seizure: A seizure could cause both syncope (loss of consciousness) and transient focal neurological deficits, including right-sided weakness. The post-ictal phase of a seizure can sometimes present with focal weaknesses.
Cardiac Arrhythmia: Conditions like supraventricular tachycardia (SVT) or ventricular tachycardia can lead to syncope. If the arrhythmia results in decreased cardiac output, it could potentially cause a transient ischemic attack (TIA) or stroke, presenting with focal neurological deficits.
Vasovagal Syncope with Injury: While vasovagal syncope itself might not directly cause right-sided weakness, if the patient fell and injured their head or neck during the syncopal episode, it could potentially lead to neurological deficits.

Subarachnoid Hemorrhage: Although less likely in a young adult without a history of hypertension or known aneurysms, a subarachnoid hemorrhage could present with sudden loss of consciousness and, if associated with aneurysmal rupture near cerebral arteries, could cause focal neurological deficits.
Cardiac Tumor or Thrombus: Conditions like cardiac myxoma or intracardiac thrombi could lead to embolic phenomena, causing both syncope (if the embolus affects cardiac function) and stroke (if the embolus lodges in a cerebral artery).
Pulmonary Embolism: A large pulmonary embolism could cause syncope due to obstruction of the pulmonary arteries and subsequent decrease in cardiac output. While less directly linked to focal neurological deficits, the hypoxia and decreased perfusion could potentially cause neurological symptoms.

Moyamoya Disease: A rare condition causing narrowing or blockage of the internal carotid artery and its branches, leading to stroke or TIA, especially in young adults. It could present with syncope and focal neurological deficits.
Mitochondrial Myopathies: Certain mitochondrial disorders can cause both cardiac and neurological symptoms, including syncope and muscle weakness, though these would typically be part of a broader clinical picture.
Arteriovenous Malformation (AVM): An AVM could cause a variety of symptoms, including seizures, syncope, and focal neurological deficits, depending on its location and whether it has bled.