This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Ohopedics Oncology, Subject name: Orthopaedics
[{'content': 'The most likely diagnosis for the tumor at upper end of tibia is:- \nSelect and explain. \nA. GCT, B. UBC, C. ABC, D. CB', 'role': 'user'}, {'content': 'Answer: A \nExplanation: * Giant cell tumor - It is the only tumor to involve the joint. It is seen in 20-40 years of age (after skeletal maturity). It occurs in epiphysis (mainly) and extends into metaphysis, so called as epiphyseal-metaphysis tumor * Unicameral bone cyst -Unilocular cystic bone cavity filled with serosanguinous fluid. X-ray shows fallen leaf sign. It often abuts but does not cross the physis. * Aneurysmal bone cyst -Blood-filled cystic spaces. X-ray shows expansile, lytic cystic lesions and MRI shows fluid-fluid levels within the lesion. * Chondroblastoma - presence of "chicken-wire" calcification is pathognomonic.', 'role': 'assistant'}]