What is the most likely cause of a reducible swelling of the scrotum in a newborn?

The most likely cause of a reducible swelling of the scrotum in a newborn is a failure of the processus vaginalis to involute. The processus vaginalis is an embryonic structure that, in normal development, should close off after the testis descends into the scrotum. When this process does not occur, it can lead to the accumulation of fluid in the tunica vaginalis, resulting in a condition known as a communicating hydrocele.

In newborns, the scrotum may present as a swelling that can be easily reduced, indicating the presence of fluid rather than solid tissue or a more complex anatomical issue. This is why the failure of the processus vaginalis to involute is often associated with a hydrocele, as fluid can move freely, creating a reducible swell in the scrotum.

Inguinal hernias and testicular torsion would likely present with more significant distress and could involve non-reducible masses, whereas a hydrocele resulting from the failure of the processus vaginalis to close is typically nonsymptomatic beyond the physical swelling noted. Thus, the characteristics of the swelling and its reducible nature support the diagnosis regarding the failure of the processus vaginalis.