3-year-old girl with sudden pain in right hip

The patient is a 3-year-old girl who presented to the urgent care center with right hip pain. She has no fever, and the parents report there was no trauma. The child has a limp and reproducible pain on external rotation of the right hip.

White blood count is 11,900 with 40.7 lymphs. The erythrocyte sedimentation rate (ESR) is 3 mm/hour. The x-ray shows fluid around the right hip joint; this finding was confirmed by ultrasound. View the image taken and consider what your diagnosis would be.