A 16-year-old girl has been experiencing severe back pain for the past two days. Despite undergoing a plain x-ray film of her spine, the results came back normal. However, two days later, she began to feel tingling sensations in her fingers and toes. The following day, she felt generally weak and decided to seek medical attention at the emergency department.
Upon examination, the girl displayed bilateral lower motor neurone facial weakness, tetraparesis with weakness in all limbs (4/5), areflexia, flexor plantars, and normal sensation. A lumbar puncture was performed, and the CSF analysis showed protein levels of 1.4 g/L (<0.4), normal glucose levels, and 0 cells (<5 ml−1). What is the most likely diagnosis?