Stroke: The most common cause of sudden onset left sided paralysis is a stroke, particularly an ischemic stroke affecting the right hemisphere of the brain, which controls the left side of the body. This is due to the high prevalence of stroke in the population and its well-known association with unilateral paralysis.

Traumatic Brain Injury: A head injury can cause damage to the right side of the brain, leading to left sided paralysis. The mechanism of injury and the presence of other symptoms such as headache, confusion, or loss of consciousness can support this diagnosis.
Brain Tumor: A tumor in the right hemisphere of the brain can cause progressive left sided paralysis, along with other symptoms like seizures, personality changes, or cognitive decline, depending on the tumor's location and size.
Multiple Sclerosis: This autoimmune disease can cause demyelination in the central nervous system, leading to a variety of neurological symptoms, including unilateral paralysis. The diagnosis is supported by a history of relapsing and remitting symptoms, and findings on MRI.

Spinal Cord Injury or Compression: Injury or compression of the spinal cord, particularly at the level of the cervical spine, can cause paralysis. This is a medical emergency requiring immediate attention to prevent further damage.
Intracranial Hemorrhage: Bleeding within the brain can cause sudden onset paralysis and is a life-threatening condition that requires immediate medical intervention.
Meningitis or Encephalitis: Infections of the brain and meninges can cause a range of neurological symptoms, including paralysis, and are medical emergencies due to their high mortality rate if not promptly treated.

Mitochondrial Myopathies: A group of rare genetic disorders that affect the mitochondria, leading to muscle weakness and paralysis. These conditions are typically progressive and may have other systemic manifestations.
Neurodegenerative Diseases (e.g., Amyotrophic Lateral Sclerosis - ALS): ALS is a progressive neurodegenerative disease that affects nerve cells in the brain and the spinal cord, causing loss of muscle control. While rare, it is important to consider in the differential diagnosis of progressive paralysis.
Conversion Disorder (Functional Neurological Disorder): A condition in which a person experiences neurological symptoms, such as paralysis, that cannot be explained by medical evaluation. Psychological factors are often involved, and the diagnosis is one of exclusion.