A 19-yeаr-оld cоllege student presents with left eаr pаin that began 3 days after returning frоm a beach vacation. She reports worsening pain when touching the ear and decreased hearing on the affected side. She denies fever, cough, or upper respiratory symptoms. Physical examination reveals erythema and edema of the external auditory canal with tenderness when the tragus is palpated. The tympanic membrane is difficult to visualize due to canal swelling. Which of the following is the most likely diagnosis?