Exacerbation of Multiple Sclerosis (MS): Given the patient's history of MS and the presentation of numbness from the armpits down along with bladder and bowel incontinence, an exacerbation of MS is the most likely diagnosis. These symptoms can occur due to demyelination in the spinal cord, which is a common feature of MS.

Spinal Cord Compression: This could be due to a variety of causes including disc herniation, tumor, or abscess. The symptoms of numbness and incontinence could be indicative of compression of the spinal cord, which needs urgent attention.
Transverse Myelitis: An inflammatory condition that affects the spinal cord across one level, leading to symptoms below that level. It can present similarly to an MS exacerbation but is a distinct entity that requires specific treatment.
Neuromyelitis Optica (NMO): An autoimmune condition that predominantly affects the optic nerves and spinal cord. While less common than MS, it can present with similar symptoms and is important to differentiate due to its distinct treatment and prognosis.

Spinal Epidural Abscess: A serious and potentially life-threatening condition that requires immediate diagnosis and treatment. Symptoms can include numbness, weakness, and loss of bladder and bowel control, similar to what the patient is experiencing.
Cauda Equina Syndrome: A condition that occurs when the nerves in the spinal canal are compressed, leading to loss of bladder and bowel control, among other symptoms. It is a medical emergency that requires prompt surgical intervention.
Spinal Hemorrhage or Infarction: Although rare, these conditions can present with sudden onset of neurological deficits, including numbness and incontinence, and are medical emergencies.

Syringomyelia: A condition characterized by the formation of a fluid-filled cavity or syrinx within the spinal cord, which can expand and elongate over time, damaging the spinal cord. It can cause a wide range of symptoms, including numbness and loss of bladder and bowel control.
Tuberculosis of the Spine (Pott's Disease): A rare condition in developed countries, it can cause spinal cord compression and present with neurological symptoms similar to those described.
Vitamin B12 Deficiency: Can lead to subacute combined degeneration of the spinal cord, presenting with numbness, weakness, and incontinence. Although less likely, it's an important diagnosis to consider due to its treatable nature.