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: Pathology
Struma ovarii is 
Select and explain. 
A. Teratoma of ovary, B. Thyroid gland tumor in pregnancy, C. May present as hypothyroidism, D. Ovarian stromal hyperplasia
A: Teratoma of ovary
Answer: A: Teratoma of ovary 
Explanation: Struma ovarii is the most common type of specialised teratoma.
It is composed of mature thyroid tissue and may present as hyperthyroidism.