Anatomy Question:

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After studying Gross Anatomy for the National Board Exams, a second year student left Sackler at 1:00am and he was mugged on Harrison Avenue. During the attack, the student held his arm up to protect his head and he was knifed on his forearm. Subsequently, while waiting to be seen in the emergency room at Nemch, the student realized that he could no longer move his thumb)

He immediately concluded that the knife must have directly injured his: