**Core Concept**
Exchange transfusion is a life-saving procedure used in neonates to rapidly reduce the levels of circulating bilirubin and other substances in the blood. It involves the gradual replacement of the infant's blood with donor blood, usually in cases of severe hyperbilirubinemia or hemolytic disease of the newborn.

**Why the Correct Answer is Right**
Cord bilirubin levels 20-25 mg/100 ml may warrant exchange transfusion, depending on the clinical context.

**Why Each Wrong Option is Incorrect**
**Option A:** Elevated unconjugated bilirubin levels > 18 mg/100 ml may indeed be an indication for exchange transfusion, as this value is above the threshold for potential kernicterus. Thus, option A is incorrect because it describes a scenario that may warrant exchange transfusion.
**Option B:** Cord hemoglobin 3.5 is actually an indicator of the risk of kernicterus, suggesting that exchange transfusion may be necessary.

**Clinical Pearl / High-Yield Fact**
When considering exchange transfusion, it's essential to remember the "rule of 20": if the total serum bilirubin is > 20 mg/100 ml, or if the infant is 15 mg/100 ml, or if the infant has a risk factor for kernicterus, exchange transfusion may be warranted.

**✓ Correct Answer: C. Cord bilirubin < 5 mg/100 ml. This option is incorrect because it describes a normal value not typically associated with the need for exchange transfusion.**