Final answer:
The 15-year-old is most likely to experience moderate dental fluorosis due to the exposure to high levels of fluoride (6ppm) in the new community's water, which exceeds U.S. EPA safety recommendations.
Step-by-step explanation:
The question concerns the potential effects on a 15-year-old who has lived in a non-fluoridated area and moves to a community where the water contains 6ppm of fluoride. Among the options given, moderate dental fluorosis is most likely to occur in this situation due to the higher levels of fluoride exposure compared to what is considered optimal for dental health. The U.S. Environmental Protection Agency sets the maximum contaminant level for fluoride in drinking water at 4ppm to prevent skeletal fluorosis, a condition caused by chronic exposure to excessive fluoride levels which can lead to serious bone problems.
A concentration of 6ppm, as posited in the scenario, exceeds this recommended level and could contribute to fluoride being stored in the bones over time but moderate dental fluorosis is more immediately probable as an aesthetic effect on the teeth from excessive fluoride during tooth development. While community water fluoridation is an effective and cost-efficient method to prevent tooth decay and promote dental health, excessive fluoride exposure, especially in children during the years of tooth development, can result in alterations in the appearance of enamel which is indicative of dental fluorosis.