Systemic Lupus Erythematosus
SLE is a pretty strange disease. It manifests itself as a photosensitivity in many cases, and can be thought of as an allergy to the sun with protean medical manifestations. When one looks at the patients (mostly women) who have the butterfly rash, what we look for is such subtle signs as whether or not the nasolabial fold is spared, the submentum is less involved than the chin, and, if there is a hand rash, whether or not the rash spares the knuckles. If it does, slide all your chips onto the "SLE" spot on the table, and expect to cash in.
In contradistinction to dermatomyositis, (see the next post) even though there is periungual erythema and perhaps telangiectasia, the sparing of the MCPJs and the PIPJs is classic SLE, whereas with DM one would expect it to involve the knuckles, the so-called Gottron's papules.
Why couldn't this be SCLE?? Well, it could, but SLE is a far better answer given the photo, because there really isn't a papulosquamous look to this at all, but more just that inflammatory look. Remember, when faced with a choice, pull the trigger on the MOST likely of the dxes.
One last point to make. Obviously, this is a systemic disease (or they would have named it something else, wouldn't they?) but one of the main systems it affects that most people forget is the brain. When their antibody titers are cranked up and they are really sick, they get a cerebritis which can make them loopy, unpleasant or, in rare cases, quasi-psychotic. So, forgive them if they are flaring and are cranky with you. They probably don't mean what they say.
Your job, above all else, is to get them to stay out of the sun. Yell at them if they disobey you. Hurt their feelings if necessary, because with each flare they get, they can further damage their kidneys, joints, etc. and those don't recover from the damage.