Three weeks after an upper respiratory illness, a 25-year-old man develops weakness of his legs, which progresses over several days. On physical examination he has 4/5 strength in his arms but only 2/5 in the legs bilaterally. There is no sensory deficit, but knee and ankle reflexes cannot be elicited. During a 2-day observation period the weakness ascends, and he begins to notice increasing weakness of the hands. He notices mild tingling, but the sensory examination continues to be normal. The workup of this patient is most likely to show which of the following?