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 composed entirely of - 
Select and explain. 
A. Mature thyroid tissue, B. Immature thyroid tissue, C. Primary ovarian carcinoid tissue, D. None of the above
A: Mature thyroid tissue
Answer: A: Mature thyroid tissue 
Explanation: Struma ovarii is a specialized teratoma which is always unilateral and is composed of mature thyroid tissue. It may manifest itself with the features of hyperthyroidism.