A 25-year-old woman has a lump in her groin that does not reduce when lying down. What is the most likely diagnosis?

The most likely diagnosis for a lump in the groin that does not reduce when lying down is indeed hydrocele of the canal of Nuck. A hydrocele occurs when fluid collects in the sac surrounding the testes; however, in females, a similar condition can arise in the canal of Nuck, which is a persistent remnant of the processus vaginalis.

In this case, the key element is the lump's inability to reduce upon lying down. An inguinal hernia, while common in the groin region, typically presents with a mass that can be pushed back (reduced) when lying down. Similarly, lymphadenopathy may present as a palpable mass, but it is less likely to be described as a "lump" in this scenario, especially without additional symptoms suggesting infection or malignancy. A varicocele is an abnormal enlargement of the veins within the scrotum and is not typically associated with a lump in the groin or specifically does not occur outside the scrotal area.

Therefore, the details aligning with a hydrocele of the canal of Nuck—specifically the characteristics of the lump and its behavior—lead to this being the most accurate diagnosis in this scenario.