The picture is kind of dark. It may be Staph (don't know about MRSA). You can't tell by looking at a photo if you have MRSA the lab has to do a culture and find out if the staph is resistant to certain antibiotics. It maybe be a Staphylococcus aureus though which the organisms that could be MRSA or a sensitve type too. Either way you should get to a doctor and have them look at it. What ever you do don't scratch. Wash any towels, and clothes that have had contact with the area and wash your hands frequently. It sounds like you spread it to your arm too. If they can't get you in try putting a triple antibiotic ointment on your arm and on the buttock area as directed on the tube. Any time you touch the area wash your hands and then clean the area you touched as well. Best to wear gloves. This is a highly contagious organism. Since you don't know for sure if it is Staph best to try to get into the doctor. It could be an allergy but usually staph looks crusty or like a boil.
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