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
Nidus is seen in: 
Select and explain. 
A. Osteoid osteoma, B. Osteosarcoma, C. Ewing's sarcoma, D. Chondroblastoma
A: Osteoid osteoma
Answer: A: Osteoid osteoma 
Explanation: Osteoid Osteoma-- mc at femur diaphysis. mc benign true bone tumor,. Typical presentation - Pain that is worse at night and is relieved by aspirin When the lesion is in a veebra, scoliosis may occur. Both osteoblastic & osteoclastic cells are seen The lesion has central lysis with surrounding sclerosis. Growing centre is called as nidus (seed). CT is the best study to identify the nidus and confirms the diagnosis.