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14yo boy presents with LLQ abdominal pain and left hip pain for 5 days. He has not had any fever. He is an active baseball player and skinned his right knee the week before. On exam, he prefers to keep his left hip flexed at 90 degrees, walks with a limp and lordosis, and is most comfortable laying on his right side with his hips flexed. He has LLQ abdominal tenderness and has maximal pain with any attempt to extend or internally rotate his left hip. Genitourinary exam is normal.

Which of the following tests is most likely to be diagnostic at this time?