This study will measure components or metabolites of PEG 3350 in blood and urine from children who are already taking PEG 3350 and compare these levels to measurements of the same molecules from a group of children who are not taking PEG 3350. PEG 3350 will not be prescribed as part of this study. Participants will complete questionnaires and provide access to their medical records. The study is being conducted in collaboration with Ohio State University and the US Food and Drug Administration.