Articles are always a problem, and there is never one answer! In your first example, the article refers to the word "doctor" and you would choose "a" or "the" depending on whether or not you are referring to a specific doctor ("a" for any doctor, "the" for a specific doctor). In your second example, most of the time you would use no article, as you are really talking about milk in general, and the word "cow's" just defines what type of milk.
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