A 6-year-old boy is brought to the office by his parents due to left hip pain and difficulty walking. The patient awoke yesterday with a limp and hip pain that exaggerates movement and improves after sleep. He plays soccer but does not recall any injuries. He went on a hiking trip a week ago, during which he experienced diarrhea and vomiting. He had no fever, abdominal pain, or other joint pain. The patient does not take any medications, and immunizations are up to date. History of rheumatoid arthritis is present in his grandfather. Temperature is 37.1 C ( 98.8 F), blood pressure is 95/65 mm of Hg, the pulse is 80/min, and respirations are 17/ min. BMI is at the 60th percentile. There is no gross abnormality or tenderness. The patient can stand and bear weight on the left lower extremity. Other systemic examination findings are normal. Laboratory results are unremarkable. Ultrasound of the left hip reveals a small fluid collection.