This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Radiology
Bone within bone appearance is seen in – 
Select and explain. 
A. CML, B. Osteoporosis, C. Osteopetrosis, D. Bone infarct
C: Osteopetrosis
Answer: C: Osteopetrosis 
Explanation: Bone within a bone appearance may be seen in osteopetrosis.