Multiple Sclerosis (MS) Exacerbation: Given the patient's history of MS, an exacerbation or relapse of the disease is the most likely cause of new or worsening neurological symptoms such as right upper extremity weakness and tingling. MS is known for its unpredictable course, and such symptoms are common during relapses.

Cervical Spondylosis or Disc Herniation: These conditions can cause compression of the spinal cord or nerve roots, leading to symptoms like weakness and tingling in the upper extremities. The likelihood increases with age and the presence of degenerative changes.
Peripheral Neuropathy: Although less common as a primary cause of unilateral upper extremity weakness, peripheral neuropathy can cause tingling and weakness. The presence of MS might increase the susceptibility to other neuropathies due to shared risk factors or disease mechanisms.
Stroke or Transient Ischemic Attack (TIA): While less likely given the history of MS, a stroke or TIA can present with sudden onset of weakness and tingling, typically on one side of the body. The risk factors for stroke might be present in patients with MS, especially those with a long-standing disease.

Cervical Spine Injury or Instability: Especially if there's a history of trauma, an injury to the cervical spine could lead to spinal cord compression or damage, presenting with similar symptoms. This is critical to identify promptly to prevent further injury.
Spinal Cord Compression from Other Causes (e.g., Tumor, Abscess): Any mass lesion in the spinal canal can compress the spinal cord, leading to neurological deficits. Early diagnosis is crucial for effective treatment.
Vascular Malformations or Aneurysms: Although rare, vascular abnormalities can cause neurological symptoms by compressing or bleeding into the spinal cord or brain, necessitating urgent intervention.

Neuromyelitis Optica Spectrum Disorder (NMOSD): An autoimmune inflammatory condition that predominantly affects the optic nerve and spinal cord, which could be considered in patients with a history of MS, especially if they have optic neuritis or longitudinally extensive spinal cord lesions.
Acute Disseminated Encephalomyelitis (ADEM): A rare, inflammatory condition of the brain and spinal cord, which can present with a variety of neurological symptoms, including weakness and tingling. It's more common in children but can occur in adults, especially after viral infections or vaccinations.
Sjögren's Syndrome or Other Connective Tissue Diseases: These autoimmune diseases can cause neurological symptoms, including neuropathy and myelopathy, though they are less common causes of focal upper extremity weakness and tingling.