Final answer:
The most crucial step when administering phototherapy to a jaundiced newborn is to protect the baby's eyes from the light, as their eyes are very sensitive and can be damaged by the phototherapy light.
Step-by-step explanation:
When administering phototherapy to a jaundiced newborn, the most important action to take is D. Protect the newborn's eyes from the light. The light used in phototherapy can be harmful to the baby's delicate eyes, so using protective eye coverings is crucial to prevent damage. While it is true that several UV photons are required to disrupt cell reproduction or kill a bacterium, the eyes are particularly sensitive and need to be shielded. The UV light in phototherapy assists in the breakdown of bilirubin in the blood, which at high levels is toxic and causes jaundice. Phototherapy is a beneficial treatment as it mimics the sunlight's ability to alter molecules such as triggering vitamin D production, which newborns can't efficiently produce on their own. However, applying sunscreen is not necessary, and increasing fluid intake is secondary to eye protection during this treatment.