How do you choose a dissertation topic?

MSc Molecular Medicine

Not by coming on here! Do you choose from a list of available projects or have to come up with your own? Either way you need to identify what interests you, do some reading around the subject and talk to potential supervisors first. You need to identify a gap in the knowledge and then a realistic way to address it using specific techniques and methods available to you.

I have an idea, but I don't know if it would fit in the course or if I'd be allowed to do it. I wanted to know how to go about deciding what to research. AFAIK, there's no list, it's discussed once the course starts

In that case discuss it with your tutor first and go from there. I'm assuming you've done some research around the topic? You then need to identify a supervisor.