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
[{'content': 'Struma ovarii is composed entirely of - \nSelect and explain. \nA. Mature thyroid tissue, B. Immature thyroid tissue, C. Primary ovarian carcinoid tissue, D. None of the above', 'role': 'user'}, {'content': 'Answer: A \nExplanation: 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.', 'role': 'assistant'}]