Fetus-in-fetu: a case report with molecular analysis.

This is the report of a case of fetus-in-fetu diagnosed in a 3-month-old boy and found to be located in the upper retroperitoneum. The entity was distinguished from teratoma by the presence of vertebral axis with limb buds. It corresponded to a diamniotic, monochorionic, monozygotic twin. Complete excision of the mass was performed. Radiological, histopathologic, and DNA fingerprinting studies performed on fetus-in-fetu specimen showed that the fetus was a monozygotic twin.