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: Gynaecology & Obstetrics
All of the following ovarian tumours usually occur bilaterally, except: 
Select and explain. 
A. Metastatic mass, B. Dysgerminoma, C. Cyst adenoma of ovary, D. Dermoid cyst
D: Dermoid cyst
Answer: D: Dermoid cyst 
Explanation: None