What is the most likely cause of jaundice in a newborn whose mother has type O Rh-positive blood and who is type B Rh-negative?

In the scenario where the mother has type O Rh-positive blood and the newborn has type B Rh-negative blood, the most likely cause of jaundice in the newborn is ABO incompatibility.

ABO incompatibility occurs when the maternal blood type is different from the fetal blood type. In this case, the mother has type O blood, meaning she has anti-A and anti-B antibodies. If the newborn inherits type B blood, the mother's antibodies can cross the placenta and attack the baby's A antigens, leading to hemolysis (destruction of red blood cells). This destruction releases bilirubin, which accumulates and leads to jaundice.

This condition is relatively common and typically results in milder jaundice compared to Rh incompatibility, owing to the presence of pre-existing maternal antibodies in ABO incompatibility cases. It often resolves on its own without significant treatment, although monitoring of bilirubin levels is important to prevent complications.

While Rhesus incompatibility can also cause jaundice, it is more serious and occurs when an Rh-negative mother carries an Rh-positive baby, which is not the case here. Genetic disorders and hyperthyroidism are less likely causes of jaundice in this context, as they do not typically result in immediate hemolytic jaundice