Are you asking whether that D would hold you back? Probably not if you've retaken the class and have done better. It might come up as a question during your interviews so prepare to have a good answer (your reason above is not good enough, there are way "scarier" classes in med school). Poor study habits is fine for a reason if you go on to clarify that you've been working hard on it ever since and are now a much better student... something like that.