Fixed Drug Eruption
This is deceptively difficult. as evidenced by the fact that none of our playas got this one right on the first swing of the bat and you're all pretty damn smart. But, throw away any preconceptions and just look at the lesion.
First, it is oval, rather than figurate or angled or irregular in border. Second, look at the border. It is flat, confluent with the surrounding skin as opposed to raised. The skin is erythematous, but evenly so. *And most important*, it is the same elevation and texture all the way across the lesion. If you add all those things up, it can only be an "inside job". Any external stimulus (contact, spider bite, etc.) would necessarily have a focal point or series of focal points.
It is well demarcated, which rules out any possibility of necrotizing fasciitis, and to rule out cellulitis, look at how oval it is, the absence of edema, and how demarcated it is. Can strep be well demarcated and oval? Yeah, but unlikely, and remember strep causes such a profound local reaction that the body tries to wall it off, causing localized lymphostasis, therefore making it swollen. Erythema Chronicum Migrans is also raised, not flat with the surrounding skin.
Finally, you have the violaceous center. which cues you in to the FDE. This is typical for an acute FDE to have a violaceous center. So, everything I just said should take about 15 seconds to process, and so then the next words out of your mouth are "have you ever had anything like this before?"