A 4-year-old boy presents with a 4-month history of bilateral hearing loss and inattentiveness in school. Otoscopy reveals dull, retracted tympanic membranes bilaterally with visible air-fluid levels. Tympanometry is performed, and the result shown as structure **A** in the diagram is obtained. Ear canal volume is normal (0.8 mL bilaterally). Which of the following is the most appropriate next step in management?