**Core Concept**
Ebstein's anomaly is a congenital heart defect characterized by an abnormal formation of the tricuspid valve and the right ventricle. The tricuspid valve is abnormally formed and the right ventricle is small and thin-walled, leading to impaired right ventricular function.

**Why the Correct Answer is Right**
The correct answer is related to the teratogenic effects of lithium, a medication used to treat bipolar disorder. Lithium is known to cause Ebstein's anomaly when taken during pregnancy, particularly during the first trimester. The exact mechanism is not fully understood, but it is thought to involve the disruption of fetal cardiac development. Lithium can interfere with the normal formation of the tricuspid valve and right ventricle, leading to the characteristic features of Ebstein's anomaly.

**Why Each Wrong Option is Incorrect**
* **Option A:** This is not a known teratogen associated with Ebstein's anomaly.
* **Option B:** This is not a known teratogen associated with Ebstein's anomaly.
* **Option C:** This is not a known teratogen associated with Ebstein's anomaly.

**Clinical Pearl / High-Yield Fact**
It is essential for healthcare providers to inform pregnant women about the potential risks of lithium on fetal cardiac development and to recommend alternative treatments if possible. Early detection and management of Ebstein's anomaly are crucial to improve outcomes for affected infants.

**Correct Answer: C. Lithium.**