Final answer:
Physiologic jaundice of the newborn is due to insufficient development of the enzyme UDP glucuronyl transferase, leading to a buildup of unconjugated bilirubin.
Step-by-step explanation:
Physiologic jaundice of the newborn is primarily due to a deficiency of the enzyme UDP glucuronyl transferase. In newborns, this enzyme is not fully developed which leads to an accumulation of unconjugated bilirubin in the blood. When the level of unconjugated bilirubin exceeds the capacity of albumin to bind it, usually over 25 mg/100 ml, it may penetrate the still-developing blood-brain barrier of the infant, potentially leading to serious conditions like kernicterus. This form of jaundice is usually managed with phototherapy, which aids in breaking down bilirubin so that it can be excreted from the newborn's body.