Torsion of the appendix testis (torsion of the hydatid of Morgagni) is the most common cause of an acute painful hemiscrotum in a child. The appendix testis is located at the upper pole of the testis (between the testis and the head of the epididymis).

An ultrasound is helpful in confirming the diagnosis.

Torsion of the appendix testis was confirmed intra-operatively.