Stroke: Given the sudden onset of unilateral deficits, stroke is a leading consideration, even in young patients, due to potential causes like cardioembolism, vasculitis, or drug use.

Multiple Sclerosis: This condition can present with a wide range of neurological deficits, including unilateral symptoms, and is a common cause of neurological disability in young adults.
Traumatic Brain Injury: Depending on the mechanism and severity, a traumatic brain injury could result in focal neurological deficits, including those isolated to one side of the body.
Intracranial Tumors: Although less common, tumors such as gliomas or meningiomas can cause localized deficits by compressing or infiltrating brain tissue.

Cerebral Venous Thrombosis: This condition, although less common than arterial stroke, can present with similar symptoms and has a high mortality if not promptly treated.
Subarachnoid Hemorrhage: Typically presenting with a severe headache, it can also cause focal deficits and is critical to diagnose early due to the high risk of rebleeding and vasospasm.
Infectious Causes (e.g., Brain Abscess): Focal neurological deficits can be a presentation of brain abscesses or other infectious processes, which are medical emergencies requiring prompt diagnosis and treatment.

Mitochondrial Encephalomyopathy, Lactic Acidosis, and Stroke-like episodes (MELAS): A rare mitochondrial disorder that can cause stroke-like episodes in young individuals.
CADASIL (Cerebral Autosomal Dominant Arteriopathy with Subcortical Infarcts and Leukoencephalopathy): An inherited condition leading to recurrent strokes and dementia, typically starting in mid-adult life but can present earlier.
Rasmussen's Encephalitis: A rare inflammatory neurological disease, often affecting one half of the brain, leading to progressive deficits including hemiparesis, seizures, and cognitive decline.