A 22-year-old female presents to the emergency department with bilateral shoulder pain, limb weakness, numbness, and difficulty urinating for 1 day, preceded by an upper respiratory infection. Examination shows: cranial nerves (-), limb tendon reflexes (++), muscle strength grade 3, sensory deficit below the shoulders and clavicles, and bilateral Babinski sign (-). Spinal X-ray and CT are normal. Lumbar puncture reveals colorless, clear cerebrospinal fluid (CSF) with WBC 0×10<sup>6</sup>/L (10/mm<sup>3</sup>), RBC 0×10<sup>6</sup>/L, protein 500mg/L (50mg/dl), and Queckenstedt test (-). The most likely diagnosis is: