X-ray<unk>  (AP & bilateral Frog leg), femur, knee
Consider CBC, ESR, CRP
Consider:
Ultrasound to evaluate for effusion
CT or MRI to evaluate for abscess
Ultrasound to evaluate for effusion
CT or MRI to evaluate for abscess
ESR > 40 mm/hr
WBC > 12,000/microliter
Refusal or inability to weight bear on affected joint
Fever 38.5° C or greater
Hip pain 
Acute rheumatic fever
Flank pain
Developmental dysplasia of hip
Lower back pain
Femur fracture
Back pain 
Juvenile idiopathic arthritis