A 13-year-old boy has a 3-day history of low-grade fever, symptoms of upper respiratory infection, and a sore throat. A few hours before his presentation to the emergency room, he has an abrupt onset of high fever, difficulty swallowing, and poor handling of his secretions. He indicates that he has a marked worsening in the severity of his sore throat. His pharynx has a fluctuant bulge in the posterior wall. A soft tissue radiograph of his neck is shown. Which of the following is the most appropriate initial therapy for this patient?