A rollerblading fall has produced a skin abrasion on the knee. the skin is broken, along with numerous small blood vessels, resulting in a small, steady ____________ of blood.

The correct answer should be loss.
When your skin is cut open, obviously a little blood will run out as a result of that. This case is no different - the person who fell and has an abrasion on the knee will experience a slight loss of blood through that tiny wound which is the result of them falling on the ground.