Final answer:
Vitamin K is given to newborns to ensure proper blood clotting as their bodies cannot produce sufficient amounts until gut bacteria develop and because breast milk often has an inadequate amount of vitamin K.
Step-by-step explanation:
A nurse would explain to the parents that their newborn is receiving vitamin K because it is essential for the proper formation of prothrombin, a blood plasma protein crucial for the blood clotting process. In newborns, particularly those that are breastfed, vitamin K levels can be low since the content in mother's milk is often inadequate unless supplemented, and the infant gut does not have bacteria to synthesize it yet. Deficiency in vitamin K can lead to increased clotting time and decreased blood prothrombin levels, resulting in potential bleeding issues, which is why supplementation is crucial in the early stages of a newborn's life.