Which of the following factors contributes to the difficulty in concentrating urine in preterm infants?

The immaturity of renal nephrons is a significant factor in the difficulty preterm infants experience when concentrating urine. In preterm infants, the renal nephrons, which are the functional units of the kidneys, are not fully developed. This immaturity affects their ability to perform key functions, such as concentrating urine.

In particular, the ability to concentrate urine is largely dependent on the development of the loop of Henle and the collecting ducts within the nephron. Preterm infants have shorter and less efficient loops of Henle, which limits the kidney's ability to create a concentrated urine as they lack the necessary anatomical and physiological maturation.

Other factors such as reduced kidney blood flow, dehydration, and excessive fluid intake can influence urine concentration, but the primary contributor, especially in preterm infants, is the developmental state of the renal nephrons. Hence, understanding nephron immaturity is crucial for recognizing the challenges faced by preterm infants in managing their fluid balance and concentrating their urine effectively.