Stephen and Martha Thompson have the birth defect.
Torsonic Polarity Syndrome, or TPS for short, is a rare birth defect seen in Season Five episode, "How to Eat with Your Butt".
This condition causes a person to grow a buttocks where their head should be, making the affected a literal "butt-head". According to Stephen and Martha Thompson, more than eleven people worldwide suffer from this condition and although some may think that they would have faces where their buttocks would normally be, this is not the case. Underneath the buttocks is a fully functional face with a full set of fully functioning organs.
The Thompson's believed Kenny McCormick's school picture prank to be their long lost child profiled on the milk carton, since Kenny is too young to be the actual lost son, the South Park Milk Company reveals that Ben Affleck is the missing boy and has undergone extensive facial operations. He is later reunited with his parents.
The disorder does not exist in real life.