An 18 year old male comes to the university clinic suppoed by his roommates because he cannot walk. He describes a rapidly evolving weakness affecting his legs and feet staing 2 days ago. On physical examination he cannot move his feet or ankles and he can barely raise his thighs off the bed. He has symmetrical hyporeflexia of the legs, but his sensorium is completely intact. Scanning his cha, the physician notes that he was treated 10 days previously for an upper respiratory tract infection. The immunological response producing the patient's symptoms is most intense at which of the following locations?

The patient has developed Guillain-Barre syndrome, also known as inflammatory polyneuropathy. This presentation is classic-rapidly evolving limb weakness with symmetrical hyporeflexia but normal sensation. The syndrome frequently follows viral infections and may evolve into complete paralysis with respiratory failure. Guillain-Barre syndrome is thought to be an autoimmune disease. The clinical course is correlated with a chronic inflammatory infiltrate and demyelination of peripheral nerves, especially spinal and cranial motor nerve roots. Inflammation localized to a small poion of spinal cord (lateral coicospinal tracts) or cerebral coex (precentral gyrus) may occur in progressive multifocal leukoencephalopathy (PML) or in multiple sclerosis (MS). PML occurs in the immunosuppressed, and MS presents with hyperreflexia (upper motor neuron signs). The classic autoimmune disease involving the neuromuscular junction is myasthenia gravis. Although the weakness caused by myasthenia gravis may affect the legs, extraocular muscles are involved in the majority of cases, and isolated limb weakness is rare. Ref: Smith M.L., Bauman J.A., Grady M. (2010). Chapter 42. Neurosurgery. In F.C. Brunicardi, D.K. Andersen, T.R. Billiar, D.L. Dunn, J.G. Hunter, J.B. Matthews, R.E. Pollock (Eds), Schwaz's Principles of Surgery, 9e.