Kennedy, a previously well 6-year old boy is brought to your general pediatric clinic with left knee pain. Upon further history taking, you elicit that the pain has lasted for 5 days. Today, Kennedy is unable to bear weight and the left knee is swollen. His mother has given him acetaminophen periodically the past few days; however it has not provided Kennedy with relief. There is no history of injury or trauma. He has a decreased appetite, periodic abdominal pain, but has not had vomiting or diarrhea. There is no history of travel. Mom says that the knee is warm to touch, but there is no redness.

What are the key diagnoses that you must rule in or rule out as soon as possible?