The Jerusalem Council

There are certain requirements to be recognized as a rabbi (teacher) within Judaism. One must be Jewish, and and ordained leader of a beit knesset with sufficient knowledge to render halakhic decisions; or an ordained Torah scholar with sufficient knowledge to render halakhic decisions.

Question: I was wondering if you could possibly shed some light on this: If a goy requires medical treatment on Shabbos, does the Halacha hold that: a) A Jewish doctor is obliged to treat him, b) A Jewish doctor is forbidden to treating him, or c) A Jewish doctor is permitted to treat him, but may refrain from doing so if he so chooses.