A 35-year-old woman has complaints of cosmetic defects of the front upper teeth crowns. The defects have been aggravating for the last 10 years. The patient suffers from unpleasant sensations when brushing her teeth, and when chemical stimuli are applied. Objective examination revealed defects localized in the enamel of the front upper teeth vestibular surface. The defects are oval, saucer-shaped, and have clear margins. Response to probing and cold stimuli was positive. Make the diagnosis: