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?

An aphthous ulcer is a common lesion that also is known as a canker sore. The lesions are never large, but are annoying and tend to occur during periods of stress. Aphthous ulcers are not infectious; they probably have an autoimmune origin. Herpetic lesions are typically vesicles that can rupture. Leukoplakia appears as white patches of thicker mucosa from hyperkeratosis. It may be a precursor to squamous cell carcinoma in a few cases. The temperance ditty mentioned in the history is a cautionary note for all young people. Oral thrush is a superficial candidal infection that occurs in diabetic, neutropenic, and immunocompromised patients. Inflammation of a salivary gland (sialadenitis), typically a minor salivary gland in the oral cavity, may produce a localized, tender nodule.