Transient synovitis

A 5-year-old girl comes to the office with a painful limp for two days. She also reports a painful right groin when moving her leg, but denies fevers. She was successfully treated for sinusitis last week. She is alert and comfortable. Her temperature is 37.3°C (99.1°F), pulse is 86/min, respirations are 18/min, and blood pressure is 120/80 mm Hg. Examination of her hips show no erythema, warmth, tenderness or joint swelling. She has an antalgic limp with a shortened stance phase on her right leg. She experiences pain with internal rotation and has a reduced range of motion in her right hip. Log roll test shows muscle guarding. Laboratory studies show a normal leukocyte count and erythrocyte sedimentation rate. Which is the most likely diagnosis in this patient?