**Core Concept**
Ectopic pregnancy is a life-threatening condition where a fertilized egg implants outside the uterus, often in the fallopian tube. Treatment involves stabilizing the patient and removing the ectopic tissue to prevent rupture and bleeding. Medical therapy with methotrexate is a common approach for unruptured ectopic pregnancies.

**Why the Correct Answer is Right**
Methotrexate is a folate antagonist that inhibits dihydrofolate reductase (DHFR), an enzyme crucial for DNA synthesis and cell division. By blocking DHFR, methotrexate prevents the rapidly dividing cells of the ectopic pregnancy from proliferating, ultimately leading to their degeneration and resolution. This mechanism makes methotrexate effective in treating ectopic pregnancies.

**Why Each Wrong Option is Incorrect**
**Option A:** **Mifepristone** is a progesterone antagonist that can be used to treat early pregnancy loss, including ectopic pregnancies, but it is not the primary choice for ectopic pregnancies.
**Option B:** **Progesterone** is often administered to support early pregnancy, but it would not be helpful in treating an ectopic pregnancy.
**Option C:** **Rho(D) immune globulin** is used to prevent Rh sensitization in pregnant women with Rh-negative blood, and it is not relevant to the treatment of ectopic pregnancies.

**Clinical Pearl / High-Yield Fact**
Methotrexate is contraindicated in ectopic pregnancies with evidence of rupture, as it can worsen bleeding and tissue damage. A stable patient with an unruptured ectopic pregnancy is the ideal candidate for medical therapy with methotrexate.

**Correct Answer: A. Mifepristone.**