A 3-week-old boy is brought by his parents to your clinic for a well-child visit.

A 3-week-old boy is brought by his parents to your clinic for a well-child visit. The newborn was born at term after an uncomplicated pregnancy. He is exclusively breastfed and is thriving. Physical examination findings are normal except for jaundice. Total bilirubin is 172 μmol/L (≤100), and conjugated bilirubin is 4 μmol/L (≥5). Results of a complete blood count and reticulocyte count are within the normal range. The results of a direct antiglobulin (Coombs) test were negative. Which one of the following, if any, is the most appropriate investigation?