This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Skeletal System, Subject name: Radiology
"Head at risk" sign is seen in - 
Select and explain. 
A. Perthes disease, B. Ewing's sarcoma, C. Osteomylitis, D. Paget's disease
A: Perthes disease
Answer: A: Perthes disease 
Explanation: Ans. is 'a' i.e., Perthes disease o "Head at risk" sign is a radiological prognostic sign for Perthes disease (also k/a Legg-Calves- Perthes disease or Coxa Plana or Idiopathic avascular necrosis of femoral head).o Disease mainly affects children o Classification of' Head at risk" sign :i) Lateral subluxation (most important prognostic factor): due to articular cartilage overgrowthii) Calcification lateral to epiphysis: calcification in the overgrown cartilageiii) Osteolysis in the superolateral part of the femoral metaphysis: Gage's signiv) Horizontal physisv) Metaphyseal cysts.