A 28-year-old man presented with a 6-day history of progressive weakness. Initially, he developed a foot drop on the left and some weakness of hand grip on the right, but over the next few days this progressed to weakness of all four limbs. He had just returned from a two-week holiday to Italy where he had spent the first few days in bed with a cough and feeling generally unwell.
On examination, there was weakness, decreased tone and areflexia in all four limbs. Plantar responses were downgoing. Cranial nerves, coordination and sensation were normal.
What is the most useful investigation to make a diagnosis in this case?