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You are seeing a 9 year old boy who returned from a camping trip in Colorado 6 days ago. He had some vague complaints of paresthesias, myalgias, and general fatigue the night before. On waking this morning, he had significant bilateral lower extremity weakness and ataxia that rapidly worsened. He is afebrile. On exam, he has no patellar or Achilles deep tendon reflexes. 

Which of the following should you look for to clinch his diagnosis?