I don't know of any successful medical startups that didn't have strong involvement (usually as founder) of an MD, usually with some experience and connections. Perhaps it's possible for a group of twentysomething techies who are complete medical-industry outsiders to produce a new disruptive product, but it's at least uncommon and makes things harder (especially to get regulatory approval and buy-in among physicians). And I'd guess the overlap between YC applicants and MDs with significant experience is quite small.