A nurse is caring for a newborn undergoing phototherapy for jaundice. How can the nurse determine that the treatment is effective? The newborn's:

Choice A rationale Phototherapy's mechanism is to change unconjugated bilirubin, a fat-soluble molecule, into water-soluble photoisomers that can be excreted via urine and bile. While successful treatment reduces lethargy and improves feeding, a decrease in feeding frequency would contradict clinical improvement and successful treatment. Choice B rationale A significant decrease in heart rate in a newborn is termed bradycardia and is often a sign of distress, hypoxia, or severe illness. Phototherapy itself has no direct therapeutic mechanism to decrease the heart rate; therefore, this physiological change is an unrelated or concerning finding, not an indicator of treatment efficacy. Choice C rationale Increased insensible water loss and frequent, loose stools are common side effects of phototherapy due to the increased rate of excretion and heat exposure. Dry, flaky skin (mild rash) is a common adverse effect, indicating the light is active, but it does not confirm the primary therapeutic goal is being achieved. Choice D rationale The goal of phototherapy is to prevent neurotoxicity from high levels of unconjugated bilirubin by facilitating its excretion. A reduction of the total serum bilirubin (normal range for full-term neonates is <5.2 mg/dL at 24 hours of age) is the definitive, quantifiable evidence that the photochemical reaction is successfully clearing the toxic metabolite.