Conductive hearing loss

A 53-year-old woman comes to the office because she thinks her hearing has worsened in her left ear. She has not noticed any discharge or felt dizzy. Her temperature is 37.2°C (98.9°F), pulse is 72/minute, respirations are 12/minute, and blood pressure is 135/80 mm Hg. A vibrating tuning fork is placed against her mastoid process, and when it is held next to her ear, she can no longer hear it. When it is placed on her forehead, she hears it more loudly on her left side. Physical examination shows no erythema or tenderness of the pinna or auricle. An otoscopic examination shows a retracted tympanic membrane and squamous debris. Which of the following is the most likely diagnosis?