**Core Concept**
Testicular teratoma in adults is a type of germ cell tumor, specifically a non-seminomatous tumor that arises from the primitive germ cells in the testes. These tumors can be benign or malignant and contain various tissue types, such as hair, muscle, and bone.

**Why the Correct Answer is Right**
Testicular teratomas in adults are malignant and can grow rapidly. They are composed of embryonic cells that have not undergone normal differentiation, resulting in the formation of various tissue types. The treatment of testicular teratoma usually involves orchiectomy, followed by chemotherapy and/or radiation therapy for malignant cases. The tumor marker beta-human chorionic gonadotropin (β-hCG) is elevated in some cases of testicular teratoma.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is likely incorrect because it does not provide any relevant information about testicular teratoma in adults.
**Option B:** This option is likely incorrect because it is not a correct classification of testicular teratoma.
**Option C:** This option is likely incorrect because it refers to a different type of testicular tumor.

**Clinical Pearl / High-Yield Fact**
Testicular teratomas in adults can be malignant and require prompt treatment, including orchiectomy, chemotherapy, and/or radiation therapy. The tumor marker β-hCG can be elevated in some cases of testicular teratoma.

**Correct Answer: D. Malignant non-seminomatous tumor.**