Multiple Sclerosis (MS): This is a common condition in young adults that can cause numbness on one side of the body due to demyelination in the central nervous system. The age of onset and the nature of the symptoms align with MS.

Migraine: Migraines can cause numbness or tingling, often on one side of the body, as part of an aura or during the headache phase. Young individuals are commonly affected by migraines.
Peripheral Neuropathy: Conditions affecting the peripheral nerves can lead to numbness. In young individuals, this could be due to diabetes, vitamin deficiencies, or autoimmune conditions like Guillain-Barré syndrome.
Stroke or Transient Ischemic Attack (TIA): Although less common in young people, strokes or TIAs can occur due to various factors (e.g., vascular malformations, drug use) and cause sudden numbness on one side of the body.

Brain Tumor: A tumor in the brain could cause localized numbness by compressing or infiltrating areas of the brain responsible for sensation. Early detection is crucial for treatment and prognosis.
Central Nervous System Vasculitis: Inflammation of the blood vessels in the brain can lead to a variety of neurological symptoms, including numbness. This condition requires prompt diagnosis and treatment to prevent serious complications.
Subarachnoid Hemorrhage: Bleeding into the space surrounding the brain can cause sudden, severe symptoms, including numbness. This is a medical emergency.

Neurosyphilis: A complication of untreated syphilis infection, neurosyphilis can cause a wide range of neurological symptoms, including numbness. Although rare, it's an important consideration in sexually active young individuals.
Sjögren's Syndrome: An autoimmune disorder that can affect the nervous system and cause numbness among other symptoms. It's less common but should be considered in the differential diagnosis, especially if other systemic symptoms are present.
Lyme Disease: Caused by a tick-borne infection, Lyme disease can lead to neurological manifestations, including numbness or tingling, in its later stages. This diagnosis is more likely in individuals who spend time outdoors in endemic areas.