A middle aged patient presents with progressive spastic paraparesis developing over a few weeks. History includes that he underwent basilar tip aneurismal clipping a year earlier for intermittent diplopia. He also describes prior bouts of numbness in the right side of his body and now also has incontinence. Recent CT head and CT angiography show the aneurismal clipping to be stable. Which of the following tests would be most helpful as a next step?
1. MRI brain and spine
2. Lumbar puncture
3. Serology for Lyme disease and immunologic screening
4. CT myelography