Final answer:
At birth, a newborn's least mature sensory system is vision. Their ability to focus and their depth perception are notably underdeveloped, unlike their hearing and smell which are more acute.
Step-by-step explanation:
The nurse caring for the newborn should be aware that the sensory system least mature at the time of birth is vision. Newborns' vision is not fully developed at birth; both the retinas and the brain parts involved in vision are immature. Newborns can focus only on things that are close, typically around 46 cm away, which is adequate for viewing the mother's face and the breast during feeding, but overall visual acuity and depth perception are limited. However, the newborn's senses of hearing and smell are comparatively more developed. They prefer human voices, especially their mother's, and can distinguish their mother's scent from others'. This demonstrates that vision requires more postnatal development compared to the other senses which are more acute at birth.