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: Surgery
Least likely to regress spontaneously is 
Select and explain. 
A. Osteosarcoma, B. Retinoblasoma, C. Choriocarcinoma, D. Malignant melanoma
A: Osteosarcoma
Answer: A: Osteosarcoma 
Explanation: Ans. is 'a' i.e., Osteosarcoma