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
[{'content': '"Head at risk" sign is seen in - \nSelect and explain. \nA. Perthes disease, B. Ewing\'s sarcoma, C. Osteomylitis, D. Paget\'s disease', 'role': 'user'}, {'content': 'Answer: A \nExplanation: 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.', 'role': 'assistant'}]