Paediatricscommon
A 13-year-old boy is brought to a general practice with three weeks of aching in his left knee and a limp his football coach has noticed. He weighs 82 kg. There has been no injury, no fever and no preceding illness, and he has been otherwise well. The left knee looks and feels normal, is not tender over the tibial tuberosity, and has a full painless range of movement. When the left hip is flexed the leg turns outwards, and he resists any attempt to rotate it inwards. Which one of the following is the most likely diagnosis?