A 40-year-old man developed tingling and numbness in the feet 2 years ago. Three months later, he noticed difficulty standing on his toes. Outside evaluation showed a small immunoglobulin G (IgG) lambda paraprotein, elevated CSF protein of 335 mg/dL (<40 mg/dL), and nerve root thickening with mild gadolinium enhancement in the cauda equina region on lumbar spine MRI. He was presumed to have Guillain-Barré syndrome (GBS) and was treated with IV immunoglobulin (IVIg).