A 3-week-old exclusively breastfed infant is brought to the clinic with jaundice persisting beyond 2 weeks of life. The mother reports that the infant feeds 8–10 times per day, but stool frequency has decreased to 2–3 times per week. Serum bilirubin is 18 mg/dL (predominantly unconjugated). Which investigation is most appropriate to differentiate breastfeeding jaundice from breast milk jaundice?