Stroke: The most common cause of sudden weakness in one arm is a stroke, particularly if the weakness is acute in onset. Given the age group and the unilateral nature of the symptoms, a stroke affecting the left hemisphere of the brain (which controls the right side of the body) is highly plausible.

Peripheral Nerve Injury: Trauma or compression of the nerves supplying the arm (such as the brachial plexus or radial, median, or ulnar nerves) could lead to weakness. This could be due to injury, repetitive strain, or conditions like carpal tunnel syndrome.
Musculoskeletal Issues: Conditions like rotator cuff injuries, tendonitis, or muscle strains could cause arm weakness, especially if the onset is gradual.
Radiculopathy: Compression or irritation of the nerve roots in the cervical spine (e.g., C5-C6 or C7) could lead to arm weakness, often accompanied by pain or numbness.

Tumors (Brain or Spinal Cord): Although less common, tumors in the brain or spinal cord could present with focal weakness. Missing such a diagnosis could have severe implications for treatment and prognosis.
Multiple Sclerosis: An autoimmune disease that affects the brain and spinal cord, leading to a wide range of neurological symptoms, including weakness. It's crucial not to miss this diagnosis due to its impact on long-term management and quality of life.
Vascular Malformations or Aneurysms: These could lead to stroke or gradual neurological deficits if they compress or bleed into surrounding brain tissue.

Amyotrophic Lateral Sclerosis (ALS): A progressive neurological disease that affects nerve cells in the brain and spinal cord, leading to loss of muscle control. ALS is rare but should be considered in the differential diagnosis of progressive weakness.
Neurosyphilis: A complication of untreated syphilis infection that can cause a variety of neurological symptoms, including weakness. Although rare in the modern era due to antibiotic treatment, it remains a consideration in certain patient populations.
Mitochondrial Myopathies: A group of disorders caused by mitochondrial dysfunction that can lead to muscle weakness among other symptoms. These are rare genetic conditions that might present with a variety of systemic and neurological symptoms.