There is a common running gag in the series House, that no matter what the symptoms of the patient are, it's never going to be lupus. Although if I remember properly in one episode it was.
Is this maybe some type of medical joke? I don't know much about medicine but I know that lupus, though not common is not that rare, so I don't think the joke makes much sense, especially in a series that shows conditions that are way-way more rare than lupus in the USA, like leprosy or genetic chimerism.