[Kromm]

This is a question of social rules more than anything else. The "doctors" of some times and places wouldn't be allowed to work as nurses in others, never mind as doctors. The "nurses" of some times and places would be better doctors than the actual doctors in others . . . but possibly not great nurses by local standards.

The best way to handle this is to create a certification represented by a License perk, and then to list the skills and levels needed to pass the exams to be awarded that License. In effect, this is similar to a martial-arts style, where you receive a perk (Style Familiarity) when you learn the right set of skills – except that unlike in that case, you'll usually need more than just one point in certain skills. But as for a martial-arts style, you could also include optional perks, skills, and techniques.

Here, License (RN) and License (MD) would be separate perks. As it takes more study to become an MD in most societies, the latter might be more prestigious – but it's still just a perk because the "real" cost is in the skills. And being an MD doesn't let you act as an RN. In fact, in most jurisdictions in the modern world, it's illegal to work as an MD and an RD on the same shift; you have to clock in and out as one or the other.

As for "What skills, then?", they're basically drawn from the same set: Diagnosis, Electronics Operation (Medical), First Aid, Pharmacy, Physician, Physiology, Psychology, Surgery, etc. Just use years of study to guesstimate total points in these things. Allocate those points as suits your society. It's very common for a modern-day nurse to end up with better Electronics Operation and Psychology that a doctor, lower Physiology than a doctor, much lower Diagnosis, and no Surgery at all.

I'll add that I think a Professional Skill would be overkill here. Most of the core work of a nurse is in Physician skill already; that's the skill of patient care. What makes doctors "better" is possibly a higher level of Physician, definitely higher levels of Diagnosis and Physiology, and at least basic Surgery. Doctors are better at deducing what's gone wrong and where.