Paediatricsmiddle twist
A five-week-old exclusively breastfed boy is brought to the general practitioner because he is still yellow. He was born at term weighing 3.4 kg and now weighs 4.5 kg. He feeds well, is alert and has a normal newborn screening result. His mother says his nappies stain a deep yellow and that his stools, dark green in the first week, are now the colour of putty. The liver edge is palpable 2 cm below the costal margin and the spleen is not felt. Which one of the following investigations will be most useful in establishing the cause?