**Core Concept**
Malignant germ cell tumors of the ovary are a type of ovarian cancer that arises from the primordial germ cells. These tumors are typically classified into three main categories: dysgerminomas, yolk sac tumors, and nongerminomatous germ cell tumors, which include choriocarcinoma, embryonal carcinoma, and teratoma.

**Why the Correct Answer is Right**
Choriocarcinoma is a type of nongerminomatous germ cell tumor that is known for its aggressive behavior and high metastatic potential. It is characterized by the presence of cytotrophoblasts and syncytiotrophoblasts, which produce human chorionic gonadotropin (hCG). Teratoma is another type of nongerminomatous germ cell tumor that can contain tissues from all three germ layers, including ectoderm, endoderm, and mesoderm. Both choriocarcinoma and teratoma are recognized as malignant germ cell tumors of the ovary.

**Why Each Wrong Option is Incorrect**
**Option B:** Arrhenoblastoma, also known as Sertoli-Leydig cell tumor, is a type of sex cord-stromal tumor that produces androgens, leading to virilization. It is not a malignant germ cell tumor.
**Option C:** Brenner's tumor is a type of epithelial ovarian tumor that is typically benign or low-grade malignant. It is not a malignant germ cell tumor.
**Option D:** Serous cystadenoma is a type of epithelial ovarian tumor that is typically benign. It is not a malignant germ cell tumor.

**Clinical Pearl / High-Yield Fact**
Malignant germ cell tumors of the ovary, particularly choriocarcinoma and teratoma, often present with elevated levels of human chorionic gonadotropin (hCG) and alpha-fetoprotein (AFP), respectively.

**Correct Answer: A. Arrhenoblastoma**