**Question:** Unilateral undescended testis around what age typically undergoes descent?

**Core Concept:** Undescended testis (UDT) is a congenital condition where one or both testes fail to descend into the scrotum from the retroperitoneal region during fetal development. The process of testicular descent is a complex interplay between hormonal and mechanical factors.

**Why the Correct Answer is Right:** In most cases, testicular descent is largely dependent on the action of the hormone chorionic gonadotropin (hCG) and its downstream mediator, inhibin B. These hormones stimulate the Sertoli cells to produce retinoic acid, which in turn stimulates the smooth muscle cells surrounding the gubernaculum to contract, leading to testicular descent. In most cases, the testis will descend by the end of the first year of life.

**Why Each Wrong Option is Incorrect:**
A. Early infancy: While testicular descent usually occurs during this period, there are exceptions, and the question specifically asks around what age the majority of testes descend.
B. Late infancy: This is too late as the testes should descend by the end of the first year.
C. Early childhood: While some testes may descend during early childhood, the majority should have descended by the end of the first year.
D. Late adolescence: This is incorrect as testicular descent is a neonatal process and should be completed by the end of the first year.

**Clinical Pearl:** Testicular descent is essential for fertility and male sexual function. Delayed descent may lead to cryptorchidism and potential fertility issues due to impaired testicular function, increased risk of testicular cancer, and hydrocele formation.

**Correct Answer:** D. Late adolescence (around 12-18 months): The majority of testes typically descend during this period, although some may take longer than two years for descent.