You can promise bone marrow donation and then, at the last moment, back off.The intended recipient will die, as their own bone marrow has been thoroughly destroyed by medications in preparation to the transplantation. But no one can force you to help them. You have a right to change your mind, even in the morning of the planned operation. You can revoke your consent anytime.But you will look like a total psychopath, if *look* is the correct word.