A distressed mother presents her 4 year old boy to the Emergency Department. She states that he appeared healthy when she put him to bed, but when she woke him he complained of intense pain and a sore neck when she lifted his head, and noticed several purple blotches on his arms and legs. She also states that he has a high fever and has vomitted twice on the way to the hospital.

Which of the following would be the most likely diagnosis?