Final answer:
Physiologic jaundice in newborns is caused by a deficiency of uridine diphosphate-glucuronyl transferase (UDP glucuronyl transferase), leading to an accumulation of unconjugated bilirubin in the blood.
Step-by-step explanation:
Physiologic jaundice of a newborn is due to deficiency of the enzyme uridine diphosphate-glucuronyl transferase (UDP glucuronyl transferase). This condition is seen in infants where the UDP glucuronyl transferase activity is not fully developed, leading to an accumulation of unconjugated bilirubin in the blood. Conditions such as Crigler-Najjar syndrome and Gilbert's disease involve issues with this enzyme. Crigler-Najjar syndrome is characterized by the absence of UDP glucuronyl transferase activity in liver cells, resulting in significantly elevated serum bilirubin levels, while Gilbert's disease entails a defect in the uptake of bilirubin by the liver due to lowered UDP-glucuronyl transferase activity.