Transient Ischemic Attack (TIA) or Mini-Stroke: Although the patient is young, TIAs can occur at any age, especially if there are underlying conditions such as hypertension, diabetes, or hyperlipidemia. The sudden onset of weakness without numbness or tingling could be indicative of a vascular event affecting the motor pathways of the brain.

Multiple Sclerosis (MS): MS can present with a wide range of neurological symptoms, including weakness. The absence of numbness or tingling does not rule out MS, as symptoms can vary widely among patients.
Peripheral Nerve Injury: Trauma or compression of the nerves supplying the arm could lead to weakness. However, the random nature of the weakness might suggest a more central cause.
Myasthenia Gravis: An autoimmune disorder that affects the neuromuscular junction, leading to fluctuating muscle weakness. The weakness can be localized or generalized and may worsen with activity.

Stroke: Similar to TIA but with persistent damage. Early recognition and treatment are crucial to minimize long-term disability.
Subarachnoid Hemorrhage: Although typically presents with a severe headache, it's essential to consider in cases of sudden neurological deficits.
Spinal Cord Infarction: Rare but can present with acute weakness. The absence of neck issues does not entirely rule out spinal cord pathology.

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 and typically presents with a combination of upper and lower motor neuron signs.
Mitochondrial Myopathies: A group of disorders caused by mitochondrial dysfunction, leading to muscle weakness among other symptoms. These conditions are rare and often have a more gradual onset.
Neurosarcoidosis: Sarcoidosis affecting the nervous system can present with a wide range of symptoms, including weakness. It is rare and diagnosis often requires a high index of suspicion.