This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Imaging of Metabolic Bone Disease, Subject name: Radiology
[{'content': 'Bone within a Bone appearance is seen in: \nSelect and explain. \nA. Osteo-necrosis, B. Osteoporosis, C. Osteopetrosis, D. Osteomyelitis', 'role': 'user'}, {'content': "Answer: C \nExplanation: Ans. (c) OsteopetrosisRef: Harrison 19th ed. /426e-4* Bone within a bone describes the bones that appear to have another bone within them.Causes: P.O.S.T-C.G.D# Paget's disease of bone# Osteopetrosis# Sickle cell disease# Thalassemia# Caffey's disease# Gaucher's disease# Growth recovery lines (after infancy)# Hypervitaminosis D", 'role': 'assistant'}]