Two Year Olds Bruise that wont go away

My two year old has had a bruise for almost three months. She has had two different x-rays neither have shown a chip or broken bone. The bruise is a knot under her skin that has stayed the same size. It doesnt seem to bother her most of the time. Occasionally she will tell me it hurts and always knows exactly where it is. The doctor wants to do exploratory surgery because she will have to be put to sleep for a MRI due to her age. The doctor has no clue what it could be. Do you think surgery is a good option?