**Core Concept**
The first trimester medical termination of pregnancy (MTP) involves the use of medications that can induce uterine contractions, prevent fetal development, or cause fetal death. The medications used in first trimester MTP must be effective, safe, and have minimal side effects.

**Why the Correct Answer is Right**
Mifepristone is a progesterone receptor antagonist that is commonly used in first trimester MTP. It works by blocking the action of progesterone, which is essential for maintaining pregnancy. This leads to the shedding of the endometrium and the cessation of fetal growth. Mifepristone is often used in combination with other medications like misoprostol to induce uterine contractions and expel the fetus.

**Why Each Wrong Option is Incorrect**
**Option A**: Methotrexate is a folate antagonist that is used in second trimester MTP, not first trimester. It works by inhibiting dihydrofolate reductase, which is necessary for DNA synthesis and cell division.
**Option B**: Misoprostol is a prostaglandin E1 analogue that is commonly used in combination with mifepristone for first trimester MTP. It causes uterine contractions and helps to expel the fetus.
**Option C**: Ethacrynic acid is a loop diuretic that is not used in MTP. Its mechanism of action involves inhibiting the sodium-potassium-chloride cotransporter in the thick ascending limb of the loop of Henle.

**Clinical Pearl / High-Yield Fact**
Mifepristone is a potent progesterone receptor antagonist that is used in first trimester MTP. It is essential to note that mifepristone must be used in combination with other medications like misoprostol to ensure effective and safe termination of pregnancy.

**Correct Answer:** A. Methotrexate is a folate antagonist that is used in second trimester MTP, not first trimester.