Parents of a 12-year-old child complain about white patches on the upper front teeth, which appeared six months ago. Objectively: chalk-like patches on the vestibular surfaces in the precervical area of the 13, 12, 11, 21, 22, 23 teeth. Their enamel is dull, pliable and rough on probing. There is a history of short-term pain caused by chemical stimuli. What is your provisional diagnosis?