A 68-year-old man with a history of myasthenia gravis presents to the emergency department with severe muscle weakness, excessive salivation, miosis, and bradycardia (HR 42/min). His wife reports he accidentally ingested an organophosphate pesticide while cleaning his farm equipment 2 hours ago. On examination, he has pinpoint pupils, muscle fasciculations, and respiratory distress. Which of the following drugs should be administered FIRST to reverse the acute cholinergic crisis?