Use of elexacaftor-texacaftor-ivacaftor ("ETI"), also known as Trikafta, is associated with weight gain in most people with cystic fibrosis (CF) who take this medication. However, some people gain more or less weight than is expected, and this can be either beneficial or problematic based on the individual's starting weight. The goal of this study is to compare factors between children and young adults with CF who have either had robust or minimal weight gain after one year of treatment with ETI. This observational study requires one study visit to CHOP that will last approximately 6 hours.