Parents of a 12-year-old child complain of white patches on the upper front teeth, which appeared six months ago. Objectively: chalk-like patches on the vestibular surfaces in the precervical region 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?