What is the defining feature of cryptorchidism?

The defining feature of cryptorchidism is undescended testes. This condition occurs when one or both of the testes fail to move down into the scrotum during fetal development or shortly after birth. Normally, the testes descend from the abdomen into the scrotum before birth, and if this process doesn't occur, it can lead to various complications such as infertility or increased risk of testicular cancer if left untreated.

The other options pertain to different conditions or issues: an enlarged prostate gland is unrelated to cryptorchidism and is more commonly associated with older males; excess fluid accumulation in the scrotum, known as hydrocele, is a separate condition that does not directly relate to the positioning of the testes; and swelling of the abdomen can be caused by a variety of factors, none of which specifically define cryptorchidism. Thus, the hallmark of the condition remains the undescended nature of one or both testes.