The pelvic x-ray shows bulging gluteal fat stripe in the right hip which is the symptomatic side, indicating the presence of a hip joint effusion. This was confirmed subsequently with an ultrasound of the affected hip.

The gluteal fat stripe is normally seen parallel to the superior aspect of the femoral neck. It can be displaced when there is a hip effusion.

Reference: https://pdfs.semanticscholar.org/08f1/81db9111d4633671f2dc26dccdc30f1d3b56.pdf