A. Calc is not required for a lot of medical schools, especially in schools moving to more holistic/non-requirement models of admissions pre-reqs. Look into your state schools and see what they require.
B. Just retake the class when you are ready, and only when you have figured out what you did wrong the first time. The last thing you want to do is plow forward into calculus, a harder class, and risk getting another D.