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A 12 year old boy with a history of ADHD on Adderall comes in for acute onset weakness of his bilateral upper arms, particularly on the right side. He had a viral URI 1 week ago with fever, cough, congestion, sore throat, from which he had recovered. For the past week, he did not take the Adderall because he had been sick and “forgot” to resume. On physical exam, his right arm is flaccid and motionless at his side, and he is unable to use it at all. He is able to lift his left arm partly against gravity, but it is weak. He has a low-grade fever of 38, and mild neck stiffness. His mental status is normal.

What is the most likely cause of his symptoms?