A 35-year-old woman presents to the Emergency department with a four-day history of worsening weakness affecting both legs. She reports difficulty walking downstairs and rising from a chair, as well as severe lower back pain. She denies any history of trauma and has not experienced any problems with her bowel or bladder. In the last 24 hours, she has also developed some facial weakness and has been unable to sing. She denies any sensory disturbance, but mentions cutting her finger on a rose trellis while gardening in the last week. She has a past medical history of psoriasis and hypothyroidism, and takes regular thyroxine. On examination, she has mild bilateral facial weakness and ptosis, as well as mild weakness of shoulder abduction and adduction. Tone and sensation are intact, but reflexes are depressed. Lower limb examination reveals marked weakness throughout, reduced tone, and absent knee and ankle reflexes. Plantar responses are mute. Investigations reveal normal blood counts and electrolytes, as well as a slightly elevated ESR. A lumbar puncture shows elevated CSF protein. What is the likely diagnosis for this patient?