Back Pain with a Normal MRI: What’s Next?

All of the following in the history and examination are consistent with cauda equina syndrome except:

The patient reattends to the ED 5 days later with an inability to walk due to a loss of balance. On examination, he has 5/5 power when sitting on a chair and reduced sensation in both legs globally. On attempting to walk, he is unable to raise his left leg and bear weight due to weakness. Rectal exam is normal and an MRI head shows only mild small vessel ischemia.

Which of the following is the least likely differential diagnosis in this patient?

On review by neurology, the patient is found to have ascending numbness in both legs as well as progressive weakness. By this time, he has also lost sensation to touch and vibration in both legs. He has absent joint position sense in both legs and has acute urinary retention. He is unable to stand due to severe weakness in both legs.

The neurology registrar suspects Guillain-Barré syndrome and performs a lumbar puncture.

Which of the following findings in cerebrospinal fluid is characteristic of Guillain-Barré syndrome?