## **Core Concept**
Ebstein's anomaly is a congenital heart defect characterized by the abnormal formation of the tricuspid valve and right ventricular hypoplasia. Certain drugs taken during pregnancy can increase the risk of this anomaly. The association between specific drugs and Ebstein's anomaly is a critical consideration in teratology.

## **Why the Correct Answer is Right**
Lithium is known to be associated with an increased risk of Ebstein's anomaly when taken during the first trimester of pregnancy. The exact mechanism is not fully understood, but it is believed that lithium may affect cardiac development. The risk is still relatively low, but it is a significant concern due to the severity of the anomaly.

## **Why Each Wrong Option is Incorrect**
* **Option A:** There is no well-established association between drug A and Ebstein's anomaly.
* **Option B:** Similarly, there is no established link between drug B and the development of Ebstein's anomaly.
* **Option C:** Drug C does not have a documented connection to Ebstein's anomaly.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that while lithium is associated with an increased risk of Ebstein's anomaly, the absolute risk remains low. However, lithium is not contraindicated in pregnancy but should be used cautiously, and the decision to continue or discontinue it should be made on a case-by-case basis, considering the mother's mental health and the potential risks to the fetus.

## **Correct Answer:** . Lithium.