**Core Concept:** Ovarian torsion is a medical emergency that occurs when the ovary twists on its vascular pedicle, leading to reduced blood supply and potential organ damage. Pregnant women are more prone to this due to hormonal and anatomical changes.

**Why the Correct Answer is Right:** Ovarian torsion during pregnancy is most commonly seen in large teratomas (C). Teratomas are germ cell tumors composed of tissues derived from three embryonic layers: ectoderm, mesoderm, and endoderm. These tumors can be either mature or immature, with mature teratomas being more common. Due to their size and complex tissue composition, teratomas can become large enough to twist on their vascular pedicle, leading to torsion.

**Why Each Wrong Option is Incorrect:**

A. Dysgerminoma is a germ cell tumor derived from primordial germ cells. Although it is a malignant tumor, it is smaller in size compared to teratomas, reducing the risk of torsion.

B. Sertoli-Leydig cell tumors (malignant and benign) are composed of Leydig cells and Sertoli cells. Although they can be large, their complex tissue composition is less likely to cause torsion compared to teratomas.

D. Ovarian cysts (adenexal masses) can be large, but they are usually simple cysts (fluid-filled sacs) without complex tissue composition, reducing the risk of torsion compared to teratomas.

**Clinical Pearl:** Teratomas should be a high suspicion diagnosis in pregnant patients presenting with acute abdominal pain, especially when the adnexal mass is large and complex. A prompt surgical intervention is crucial to prevent complications like infarction and rupture of the twisted ovary.

**Correct Answer:** C. Mature teratoma