You ask the paediatrician about the baby’s history and discover:

• she has never had a blood tranfusion
• mum does not have an allogenic antibody
• there is no known family history of haemolysis
• there are signs of jaundice

Further tests show

Reticulocyte count 436 (reference range for age 50-100)

Direct antibody test is negative

What is the most likely differential diagnosis and what further tests could be done to make the diagnosis?