Which characteristic of the renal system is true for a newborn?

The characteristic most accurately describing the renal system of a newborn is the statement regarding decreased renal blood flow. In neonates, especially preterm infants, the renal system is still developing, and the kidneys have not yet reached their full functional capacity that is seen in older children and adults.

At birth, the glomerular filtration rate (GFR) is lower than that of older children and adults. While it does increase within the first few days or weeks of life, the initial state is characterized by reduced filtration capacity. The renal blood flow in newborns is also generally lower compared to that of older infants and children, which contributes to less efficient renal function during the early neonatal period.

Other characteristics, such as fully matured tubular function and the ability to effectively concentrate urine, do not apply to newborns. While the tubular system develops over time, it’s not fully matured immediately after birth. Similarly, the ability to concentrate urine is limited in neonates, as their kidneys are still adapting to extra-uterine life. Therefore, the accurate characteristic for a newborn's renal system is the decreased renal blood flow, reflecting the immaturity of renal function during this stage of development.