A 22-year-old woman presents to the emergency department with bilateral shoulder pain, limb weakness, numbness, and difficulty urinating for 1 day, following a recent upper respiratory infection. Examination shows: cranial nerves intact, limb tendon reflexes (++), muscle strength grade 3, sensory deficit below the shoulders, and bilateral negative Babinski sign. On day 2, her condition worsens with muscle strength grade 0 in all limbs, absent tendon reflexes, decreased muscle tone, bilateral negative Babinski sign, and urinary retention. This presentation is now consistent with: