What kind of rash spreads from the groin?

I worked 3 in a row one week in September.  When that happens, I don't see the kids at all for 72 hours.  When I came home on day 2, Tyler told me that Ethan had a little rash in his groin folds.  He described it as mottled and flat.  I guessed it was probably heat or diaper rash (it's still hot here in September, remember?), and to just tell Holley to try to keep him really dry.  By the end of day 3, Tyler called me and told me the rash had spread.  Spread where?  Um, his whole body.

I couldn't get a really good picture of it because he was all squirmy, but the first picture is his back, and this is his belly.  It was all over his trunk and arms and legs, up onto his occiput and ears but not his face, hands, or feet.  It wasn't in his mouth/lips or eyes.  It was splotchy and slightly raised, but it didn't seem to bother him at all; he didn't cry if you touched it and wasn't really extra crabby, just normal crabby.  He didn't seem to be scratching and he didn't have a fever.

I knew it wasn't measles, rubella, impetigo, scarlet fever, or chicken pox.  It probably wasn't dermatitis because we didn't change anything soap-wise.  He hadn't been stung by a bee or eaten anything out of the ordinary, plus he wasn't having any itching or swelling, so I didn't think it was an allergy.

I decided to take him to the doctor to see what they thought.  Because we go to the pediatrics center at the UofAMC, we have to be seen by a resident before we get to see the attending.  I was lucky enough to get a first-year.  (Side note, I'm always amazed at how little the first-year residents know, what they heck do they teach you in medical school?  Also, they are very awkward talking to patients and asking the nurses for things.  Don't they practice that?  Along the same lines, I'm amazed at how "smart" they become by the time they are fourth-years.)  I went through the whole thing about how I didn't think it was A, B, C or X, Y, or Z, but I wanted to come in mostly because it was Friday, and I didn't want it to turn into something crazy over the weekend and have us end up in the ED.

The resident mostly just agreed and said he had no idea what it was.  He brought the attending in, and she was a little smoother in saying that sometimes kids get "little viruses" that manifest like this, but it would be difficult to tell exactly which virus he had.  She predicted that it would spontaneously clear in 48 hours.

And it did.  It went as fast as it came.

Crazy, huh?