A 33 year old woman has subacute onset of weakness. A week ago, she felt tired, but starting 3 days ago, she had difficulty
climbing stairs, and today she can't walk around her apartment. Her motor examination is notable for 2/5 strength in proximal legs
and 0/5 strength in distal legs, with absent reflexes at the patella and Achilles. Which test would help you confirm your clinical suspicion of the diagnosis?
Select one of the following: