A 24-year-old woman presents with foot and leg weakness that is progressively getting worse over the past 1 week. Initially, she developed tingling in her feet and noticed that they would drag, but now she has difficulty standing and walking due to the leg weakness. Three weeks ago she had a "chest cold," which resolved on its own. On examination, muscle bulk is normal, motor strength is two out of five in the quadriceps, and one out of five in the feet. Reflexes at the ankle and knee are absent, and sensation testing is normal. The upper limb examination is normal. The CSF protein is very high, glucose is normal, and cell count is slightly elevated. Which of the following is the most likely diagnosis?