A 17-year-old girl notices a small, sensitive, gray-white area forming along the lateral border of her tongue 2 days before the end of her final examinations. On examination by the physician's assistant, the girl is afebrile. There is a shallow, ulcerated, 0.3-cm lesion with an erythematous rim. No specific therapy is given, and the lesion disappears within 2 weeks. The history shows that the girl does not use tobacco or alcohol. Which of the following is the most probable diagnosis?