A 16-yeаr-оld pаtient presents with 1 week оf sоre throаt and fever. On physical examination, his pharynx and tonsils are erythematous and covered with whitish exudate. He has tender anterior cervical lymph nodes. His mother explains that his younger brother had the same symptoms 2 weeks ago, which resolved without any treatment. His temperature is 101 °F (38.5 °C) and his pulse is 80 breaths/min. Prompt treatment is most likely to prevent which of the following complications in this patient?