A 45-year-old farmer is brought to the emergency department in severe respiratory distress 30 minutes after accidental exposure to an organophosphate pesticide. He presents with profuse bronchorrhea, bronchospasm, bradycardia (HR 42/min), pinpoint pupils, and muscle fasciculations. The drug marked **A** in the diagram is immediately administered intravenously. Which of the following best describes the mechanism by which this drug reverses the cholinergic crisis?