This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Musculoskeletal system, Subject name: Anatomy
Struma ovarii is composed of 
Select and explain. 
A. Mature thyroid tissue, B. Immature thyroid tissue, C. Primary ovarian Carcionoid tissue, D. None
A: Mature thyroid tissue
Answer: A: Mature thyroid tissue 
Explanation: It is a special teratoma which is always unilateral and is composed of mature thyroid tissue