As for a Knowledge check related to medicine...presuming that you don't want to introduce a new skill, my recommendation would be that for any given creature, the Knowledge check to know about medicine as it relates to that creature is the same Knowledge check you'd make to identify them. So if you wanted to know about medicine as it relates to animals, that'd be Knowledge (nature), whereas humanoids would be Knowledge (local), and dragons are Knowledge (arcana), etc. It's not necessarily a perfect solution, but it should serve well enough, and makes it so that you don't have a single skill representing medical knowledge of all the disparate creatures in existence.