A 45-year-old farmer presents to the emergency department 2 hours after accidental exposure to parathion during pesticide spraying. He exhibits severe salivation, bronchospasm, miosis, muscle fasciculations, and bradycardia. The structure marked **D** in the diagram is the site of irreversible inhibition by the organophosphate compound. Which of the following is the MOST appropriate FIRST-LINE pharmacological intervention to manage his acute muscarinic symptoms?