**Core Concept**
The medical termination of early pregnancy involves the use of various pharmacological agents that act on the uterus to induce abortion. This process is governed by the principle of uterine contractions and cervical dilation, which can be achieved through the administration of specific medications.

**Why the Correct Answer is Right**
Mifepristone (RU-486) is a progesterone receptor antagonist that blocks the action of progesterone, leading to the degeneration of the endometrium and the cessation of embryonic development. Methotrexate is a folate antagonist that inhibits dihydrofolate reductase, disrupting DNA synthesis in rapidly dividing cells, including the embryo. Misoprostol is a prostaglandin E1 analogue that induces uterine contractions and cervical dilation, facilitating the expulsion of the products of conception.

**Why Each Wrong Option is Incorrect**
**Option A:** Mifepristone is indeed used in medical termination of early pregnancy, making it an incorrect option for this question.
**Option B:** Methotrexate is also used in medical termination of early pregnancy, particularly in cases of ectopic pregnancy, making it an incorrect option for this question.
**Option C:** Misoprostol is commonly used in conjunction with mifepristone or methotrexate for medical termination of early pregnancy, making it an incorrect option for this question.

**Clinical Pearl / High-Yield Fact**
It is essential to note that the medical termination of early pregnancy should only be performed under the guidance of a qualified healthcare provider, and the chosen regimen should be carefully selected based on the gestational age and individual patient factors.

**Correct Answer: D.**