Fetus-in-fetu: a rare entity.

Fetus-in-fetu is a malformed parasitic monozygotic diamniotic twin that is found inside the body of the living child or adult. A 3-year-old male child presented with a retroperitoneal mass, which on radiological examination showed retroperitoneal teratoma. Pathological examination revealed fetus-in-fetu with partially developed vertebral column.