Judy Patterson is a 22 year-old university student who presented to the Student Health Clinic with a rash on her lower legs. Her past medical history is unremarkable except for a urinary tract infection diagnosed 6 days ago for which she is taking trimethoprim-sulfamethoxazole. The only other medication she takes is the occasional dose of ibuprofen for headaches. She has never had any dental extractions or surgeries. On examination, she has no lymphadenopathy or splenomegaly, but she does have petechiae on her lower legs. You ask to look inside her mouth and there you see a blood blister on the inside of her cheek. She says she must have bitten it by accident.