History of right lower quadrant abdominal pain

A 24-year-old woman comes to the physician because of right lower quadrant abdominal pain. She has had the pain off and on for the past month, but it is now increasing. She has no other symptoms and no medical problems. Examination reveals a mildly tender, right adnexal mass. Pelvic ultrasound shows a 7 cm right adnexal complex cyst. Urine hCG is negative. The patient is taken to the operating room for laparotomy and right ovarian cystectomy. Microscopically the cyst has cartilage, adipose tissue, intestinal glands, hair, and a calcification that appears to be a tooth. There is also a large amount of thyroid tissue. Which of the following is the most likely diagnosis?

a)Corpus luteum
b)Ectopic pregnancy
c)Gastric carcinoma
d)Struma ovarii
e) Thyroid carcinoma

Ans: d)Struma ovarii