Firstly, I believe (although I may be wrong) that universities do not look at the individual exam marks for all your gcse papers and instead just look at your overall grades to try and see whether you would be a suitable candidate for the course and have a high chance of doing well. However, during the university application process you do have the opportunity to express any circumstances that may have affected your as results or your exam performance so you can explain about your illness and why you are resisting. If your school also knows about this they will be able to advise you on how to show this on your application. If you can do this for your as results there may also be a chance you can do it for your gcse grades and explain your circumstances for those too but I'm not sure. Overall don't panic, if circumstances that were not your fault, not predictable or not preventable then I believe most universities will take this into account and not view you negatively for resisting. So long as you get the top grades when you do resist and show a real passion for medicine you should be fine. Good luck