Anonymous4: That's the iliac crest, the anterior superior portion. I'm surprised people don't notice that they have them, but girls (especially thin ones) tend to have ones that protrude more. I would have to say not even recognizing that there is bone there would be a good sign of a virgin.
Anonymous5: ^ And someone not realizing that the bone wouldn't protrude at that position is a good sign that they failed anatomy; she's bent the wrong direction for those bones to protrude that much!
Anatomy fail.