What Does A Dark Spot Under The Arm Indicate?

My daughter has a really dark spot under her left arm. It looks like a bruise but its not. She said its been there for a while. It doesnt bother her at all but dr wants to biopsy it. I. hope just as a precaution. Diabetes runs in our family could it be releated to that and should I be worried. She said it doesnt hurt and is not sore