A 6-month-old infant is referred to the growth clinic because his weight gain has slowed significantly over the past 2 months. Birth weight was 3.2 kg, and at 3 months he weighed 5.8 kg. Current weight is 6.5 kg (below 5th percentile for age), and length is 63 cm (3rd percentile). Head circumference is 41 cm (10th percentile). The infant is exclusively breastfed and appears clinically well with normal developmental milestones. Which investigation is most appropriate to assess the adequacy of milk transfer and rule out breastfeeding-related failure to thrive?