Mechanism of action of an oral contraceptive containing a combination of oestrogen-progestin is the following except-a) Inhibition of ovulationb) A change in the cervical mucusc) Inhibition of motility of the uterine tubesd) Inhibition of motility of spermse) Changes in the uterine endometrium

**Core Concept**
The mechanism of action of oral contraceptives containing a combination of estrogen-progestin involves multiple pathways to prevent pregnancy. These pathways include inhibition of ovulation, alteration of the cervical mucus, and changes in the uterine endometrium.

**Why the Correct Answer is Right**
The correct answer involves a combination of mechanisms to prevent pregnancy. **Inhibition of ovulation** (Option A) is the primary mechanism of action of estrogen-progestin oral contraceptives, which suppresses the release of gonadotropin-releasing hormone (GnRH), leading to decreased follicle-stimulating hormone (FSH) and luteinizing hormone (LH) levels. This results in the inhibition of follicular growth and ovulation. **A change in the cervical mucus** (Option B) makes it difficult for sperm to penetrate the cervix, further preventing fertilization. **Changes in the uterine endometrium** (Option E) make implantation of a fertilized egg unlikely.

**Why Each Wrong Option is Incorrect**
**Option C:** Inhibition of motility of the uterine tubes is not a recognized mechanism of action of estrogen-progestin oral contraceptives.
**Option D:** Inhibition of motility of sperm is not a direct effect of estrogen-progestin oral contraceptives.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that oral contraceptives can have both progestin-only and estrogen-progestin formulations, with different mechanisms of action and side effect profiles.

**Correct Answer:**
B. A change in the cervical mucus