For once, it really was the one-armed man... sorry....

A one-armed teenager managed to transform a Center City mall's extra-large bathroom stall into his very own pedophile haven where, cops say, he sexually assaulted his 6-year-old niece on Monday.

The little girl - clad in a navy- blue uniform - lay stiff on the floor as her uncle knelt below her with his face between her legs, witnesses said.

His jeans, the witnesses said, were pulled down around his thighs.