Derek has a mysterious rash on his hand. It actually looks more like a burn except that he didn't burn himself. I tried putting cortisone cream on it (because men don't like to admit that they might need medication) but that didn't help.
Derek emailed me something about this plant called Hogweed that reacts with the sun and causes a chemical burn. You can see we were grasping at straws here. As I'm googling images for Hogweed skin reactions I see a few pictures of people who had a reaction on their skin to lime juice and sun. I then remembered that before this mysterious rash appeared we had been at Todd and Meg's lake house drinking Corona with limes. Looks like we won't need Dr. House after all.
Diagnosis: Corona injury or "Mexican beer dermatitis"! I hope this will save someone from a similar Mexican beer fate.