**Core Concept**
Ectopic pregnancy is a life-threatening condition where a fertilized egg implants outside the uterus, typically in the fallopian tube. Medical treatment aims to terminate the pregnancy and preserve the fallopian tube. Methotrexate is a key drug used in the management of ectopic pregnancy due to its ability to inhibit dihydrofolate reductase, leading to the death of the rapidly dividing trophoblastic cells.

**Why the Correct Answer is Right**
Methotrexate works by inhibiting dihydrofolate reductase (DHFR), an enzyme crucial for the synthesis of tetrahydrofolate, a precursor to DNA and RNA synthesis. In the context of ectopic pregnancy, this leads to the death of the rapidly dividing trophoblastic cells, thereby terminating the pregnancy. The effectiveness of methotrexate in treating ectopic pregnancy is well-established, with success rates ranging from 80-95% in selected cases.

**Why Each Wrong Option is Incorrect**
**Option B:** Progesterone is actually a hormone involved in the maintenance of pregnancy, not its termination. Administering progesterone would be counterproductive in the context of an ectopic pregnancy.
**Option C:** Oestrogen is another hormone involved in the menstrual cycle and pregnancy, but it is not used to treat ectopic pregnancy. In fact, oestrogen therapy could potentially exacerbate the condition.
**Option D:** Adriamycin, also known as doxorubicin, is a chemotherapy agent used to treat various types of cancer, but it is not typically used to treat ectopic pregnancy. Its use would be associated with significant side effects and is not a preferred treatment option.

**Clinical Pearl / High-Yield Fact**
In cases of ectopic pregnancy, the American College of Obstetricians and Gynecologists (ACOG) recommends methotrexate as a first-line treatment for selected patients, including those with a viable ectopic pregnancy, a small ectopic mass, and no evidence of tubal rupture.

**✓ Correct Answer: A. Methotrexate**