You are seeing a 46-year-old male in A&E who has presented with lower limb weakness. During the history, the patient informs you that a feeling of weakness and numbness started in his feet two days ago and the feeling has now spread up his legs. He also informs you that he was in hospital for pneumonia 3 weeks ago. During a neurological examination you note hypotonia, reduced reflexes and fasciculations in the lower limbs bilaterally. What is the most likely diagnosis?