**Core Concept**
The concept of 'X' risk in pregnancy refers to medications that are contraindicated in pregnancy due to potential severe harm or risk to the fetus. These medications can cause birth defects, miscarriage, or other serious complications.

**Why the Correct Answer is Right**
The correct answer is a medication that has been extensively studied and proven to cause significant harm to the fetus if taken during pregnancy. **Teratogenic effects** are a major concern, as these medications can disrupt the normal development of the embryo or fetus. The **fetal risk** associated with this medication is so high that it is classified as 'X' risk.

**Why Each Wrong Option is Incorrect**
**Option A:** This medication may have some teratogenic effects, but it is not classified as 'X' risk. It is still used in pregnancy under close monitoring, but the risk is not as severe as 'X' risk medications.
**Option B:** This medication is actually beneficial in pregnancy and is often used to treat conditions such as hypertension. It is not contraindicated in pregnancy and is not classified as 'X' risk.
**Option C:** This medication has some potential risks in pregnancy, but it is not as severe as 'X' risk. It may require dose adjustments or close monitoring, but it is not contraindicated.

**Clinical Pearl / High-Yield Fact**
When prescribing medications to pregnant women, always check the FDA pregnancy category and follow the guidelines. Remember that 'X' risk medications are contraindicated in pregnancy, while 'A' risk medications are safe to use.

**Correct Answer:** C. Warfarin