What is an alternative expression to "this"?

Solution 1:

There do exist alternatives:

Medicine is very important. Such can be ascertained from the fact that...

Medicine is very important. As much can be ascertained from the fact that...

Medicine is very important. That be ascertained from the fact that...

Medicine is very important. So obvious a conclusion can be ascertained from the fact that...

And so on. Be creative, don't let yourself be boxed in by a single word.

Solution 2:

Oh dear, I don't think there exists any word close enough to be considered a synonym. The closest words would be "that" and "it", but they do not fit in with the rest of your example very well. I can think of a few ways to rephrase your example:

Medicine is very important, which can be ascertained from...

Medicine is very important. The fact that ... proves its importance.

Why exactly do you want to do away with this "this"? It looks fine in your example. A bit more context (purpose?) might get you better suggestions. Is this a real sentence, or just an example?

Solution 3:

The most succinct way that I can conjure to rephrase your connected pair of statements would convert the second statement into a dependent clause of the first. E.g.:

Medicine is very important, as proved by...