Paediatricscommon
A 12-year-old boy who is on the 97th centile for weight has had three weeks of aching in his right knee and a limp that his parents noticed because his right foot "turns outward when he walks". He is afebrile and well. Examination of the knee is normal. When the right hip is flexed, the leg drifts into external rotation; internal rotation of the hip is markedly reduced and painful compared with the left. There is no history of injury.
Which one of the following is the most likely diagnosis?