A 50-year-old male presents with ascending weakness. He first noticed that he was tripping over more easily, but now has trouble getting out of a chair. He feels otherwise well. Of note, he did have mild diarrhoea which had completely resolved a week prior to developing this weakness.
On examination, he is haemodynamically stable with a heart rate of 70 beats per minute and a blood pressure of 130/80 mmHg. His respiratory rate is 16 breaths per minute. He has reduced power in ankle plantar and dorsiflexion bilaterally, absent ankle jerks and reduced knee jerks. His plantar responses are downwards.
Which of the following results will most assist you with a diagnosis in this acute presentation?