4. Rett syndrome is a disorder of the nervous system that leads to skeletal developmental problems, especially in the rate of growth of head, hands, and feet. Rett syndrome occurs almost exclusively in girls. The data in the table below present information on the average height, weight, and body mass index (BMI) for females with Rett syndrome in different age ranges. The z-scores indicate the placement of the average relative to non-Rett syndrome females in the same age range. Age Interval 0-