Lupus is kind of lousy for the punchy scientific format because it has as a nickname "The Great Imitator." (Note that the porphyrias are known as the "little imitator.") Because of this somewhat Protean nature, lupus can masquerade as quite a lot of things, leading it to be a casual element one would have to bring up and then eliminate in episode after episode. Thus, for screenwriting reasons, it needs a kind of general dismissal which would stick in the mind of viewers ...