Informed consent does not mean that you personally know everything that went into the diagnosis--everyone consulted, every test done, every theoretical underpinning for it, etc. What it means is that the diagnosis, the risks, the costs, the prognosis, etc is all divulged to you, and in light of that knowledge of the pros, cons, etc., you consent to the treatment. You can refuse to grant consent if for any reason you mistrust what you are told or feel uncomfortable with the proposed course of treatment--consent is something you must voluntarily agree to. But if you do choose to consent, you can do so without having the information you describe.