Question

49yo man presents to his primary provider’s office with a 2-week history of bilateral leg weakness. He denies any pain associated with it and has never had any symptoms like this before. He denies any previous back problems and reports never remembers injuring his back. He thinks it first started in his feet when he noticed he was dragging is toes when walking, but now finds some difficulty lifting his legs when going up stairs. PMH is significant for hypertension (controlled on lisinopril) and osteoarthritis (controlled with exercise and celecoxib). He also reports having a pretty severe case of “food poisoning” a month ago when vacationing in the gulf, but is otherwise healthy.

Physical examination reveals 3/5 strength bilaterally with plantarflexion and dorsiflexion of the ankles and 4/5 strength bilaterally with hip and knee flexion. His ankle deep tendon reflex is absent and knee is diminished at 1+. Sensation and two-point discrimination of the feet are intact

1. What would be the next step in the diagnostic evaluation of this patient?
2. What is the most likely diagnosis and cause of this disease?