Review Question - QID 214821

A 42-year-old man presents to the emergency department complaining of bilateral lower extremity weakness. He reports progressively worsening weakness over the past 2 days that initially began in his feet. The patient reports experiencing some paresthesias and pain in his legs about 2 weeks ago. He denies any lower extremity or back pain, fever, chills, urinary or fecal incontinence, or recent trauma. His medical history is significant for poorly controlled diabetes, for which he takes metformin inconsistently. His temperature is 98.6°F (37.0°C), blood pressure is 110/55 mmHg, pulse is 87/min, and respirations are 15/min. Physical examination is significant for track marks on his left arm, decreased strength in the bilateral lower extremities, and absent bilateral patellar and Achilles deep tendon reflexes (DTRs). Cranial nerves II-XII are intact, heel-to-shin and finger-to-nose are normal, and his gait is wide-based. Which of the following is the best diagnostic test for this patient’s condition?