**Core Concept**
Ectopic pregnancy is a life-threatening condition where the fertilized ovum implants outside the uterus, usually in the fallopian tube. Medical treatment of unruptured ectopic pregnancy involves administering medications to stabilize the patient and prevent rupture.

**Why the Correct Answer is Right**
The correct answer is **methotrexate (MTX)**. MTX is a folate antagonist that inhibits dihydrofolate reductase (DHFR), an enzyme essential for DNA synthesis. In the context of ectopic pregnancy, MTX prevents the proliferation of the trophoblast cells, allowing the body to resorb the ectopic pregnancy. However, MTX is contraindicated in certain situations where the risk of treatment failure or toxicity is high.

**Why Each Wrong Option is Incorrect**
**Option A:** **β-hCG levels** are not a contraindication for MTX treatment. Instead, high β-hCG levels may indicate a larger ectopic pregnancy, which may require surgical intervention.

**Option B:** **Rupture** is a complication of ectopic pregnancy, not a contraindication for MTX treatment. In fact, MTX is often used to prevent rupture in unruptured ectopic pregnancies.

**Option C:** **Age** is not a direct contraindication for MTX treatment. However, older patients may have a higher risk of treatment failure or toxicity.

**Clinical Pearl / High-Yield Fact**
When considering medical treatment for unruptured ectopic pregnancy, it's essential to assess the patient's β-hCG levels, uterine size, and the presence of any symptoms suggestive of rupture.

**Correct Answer: D. Methotrexate is contraindicated in cases of known or suspected fetal cardiac activity, as it can cause fetal cardiac toxicity.**