Hi ..my 5 mo baby boy has a recurring fluid filled blister on the back of his head since he was 2.5 months.It has happened thrice.Once the blister pops, the fluid gets drained,a brown crust forms and the there is a white residual skin left.His dermatologist has done an USG and ruled out any skull defects or underlying connections of the lesion and diagnosed it as Aplasia Cutis. I need to know how it can be proved if its really aplasia cutis as its a very rare genetic disorder as baby doesnt exhibit any symptoms at birth and he has hair follicles and hairs over the lesion too.