A 5-year-old child who has not had routine pediatric care develops a febrile disease with cough and a blotchy rash, and is brought to the emergency room. On examination, there is lymphadenopathy. Erythematous, maculopapular rash behind the ears and along the hairline, involving the neck and, to a lesser extent, the trunk. Examination of this patient's oropharynx would likely reveal which lesion?