You choose five courses on the UCAS form but only four can be medicine. The other needs to be something that makes sense in terms of your personal statement: Biomedical Sciences, for example. You can put something unrelated to medicine down (law, accountancy, etc) but the application will go in the bin when they see your Personal Statement which (presumably) says that you have thought of nothing but becoming a doctor since you first saw Casualty on the television when you were six years old.