The most important thing that your teachers will be looking for as you make your choices is evidence: either evidence that you are good enough to take the subject at advanced level, or evidence that you are interested enough in a subject to take it at advanced level if you have not studied it before. Another factor to consider if you are aiming for incredibly competitive courses at university, such as Medicine, is that you may require a very high performance in standard level qualifications. Does your performance to date match your ambition? It is important that your decisions are taken on.