**Core Concept**
Ectopic pregnancy is a life-threatening condition where the fertilized egg implants outside the uterus, most commonly in the fallopian tube. Medical management is preferred over surgical intervention to reduce morbidity and mortality.

**Why the Correct Answer is Right**
Methotrexate (MTX) is a first-line medical treatment for ectopic pregnancy, particularly when the embryo is less than 3.5 cm in size and the patient is hemodynamically stable. MTX works by inhibiting dihydrofolate reductase, an enzyme essential for DNA synthesis and cell division. This leads to the death of rapidly dividing cells, such as the ectopic embryo. Actinomycin-D is an alternative for patients with a failed MTX response or those with a larger ectopic mass.

**Why Each Wrong Option is Incorrect**
**Option A:** Actinomycin-D is a correct option, but not the only correct one.
**Option B:** HCG is a hormone produced during pregnancy, but it is not used to treat ectopic pregnancy.
**Option C:** RU-486, also known as mifepristone, is an antiprogestin used for medical abortion, not ectopic pregnancy treatment.
**Option D:** KCl is not a standard treatment for ectopic pregnancy.

**Clinical Pearl / High-Yield Fact**
In patients with ectopic pregnancy, a failed MTX response is often associated with a larger embryo size or delayed presentation. Always consider surgical intervention in such cases.

**Correct Answer:** A. Methotrexate (MTX)