Final answer:
An exchange transfusion is often required when total bilirubin levels exceed 25 mg/100 ml (or >25 mg/dL) to prevent kernicterus in newborn infants with conditions like Crigler-Najjar syndrome, where UDP glucuronyl transferase activity is deficient.
Step-by-step explanation:
The question relates to the threshold bilirubin level at which an exchange transfusion is considered necessary to prevent kernicterus, a type of brain damage that can result from high levels of unconjugated bilirubin in newborn infants. In cases of severe unconjugated hyperbilirubinemia, where the UDP glucuronyl transferase activity is underdeveloped, unconjugated bilirubin accumulates in the blood. If this level exceeds the binding capacity of albumin, which is approximately 25 mg/100 ml, it may penetrate the blood-brain barrier and cause kernicterus. Therefore, the correct answer is d) > 25 mg/dL as the level requiring exchange transfusion. This information is particularly relevant to conditions like Crigler-Najjar syndrome and Gilbert's disease, where there is defective UDP glucuronyl transferase activity.