A 2½-yr-old boy presents with an upper respiratory tract infection and axillary temperature of 38.5oC. He has not complained of ear pain. On pneumatic otoscopy, his left eardrum is pink, translucent, and retracted, with 3+/4+ mobility; his right eardrum is reddish, opaque, retracted, and immobile. Which of the following is the likeliest ear-related diagnosis?