Paediatricsmiddle twist
A 13-year-old boy is brought to the emergency department with six weeks of ache in the left knee, worse after football and now making him limp. There has been no injury, no fever and no swelling of the knee. His weight is above the 97th centile for age. The knee looks normal and moves fully without tenderness. When the left hip is passively flexed, the thigh rolls outwards. Which one of the following investigations is most likely to establish the cause of his limp?