Agenesis of the penis, scrotal raphe, and anus in one of monoamniotic twins.

Severe defects in the development of the caudal axis are rare, but lead to profound consequences for the fetus. They result from damage to multiple germ layers at a very early stage of development and vary in severity depending on the timing and degree of insult incurred. One of a set of identical male twins presented with persistence of the primitive cloaca with an absent phallus and anal atresia, but normal limb development. This combination of penile agenesis, imperforate anus, and absence of the scrotal and perineal raphae is an ominous physical finding indicative of severe renal anomalies, which in all cases have been incompatible with extrauterine survival. This constellation of findings, of which our patient is only the sixth reported example, probably results from a primary defect in caudal mesoderm migration before the fourth week of gestation. In this case, the defect may have occurred as a consequence of monochorionic monoamniotic monozygotic (MZ) twinning.