**Core Concept**
Ergometrine is an ergot alkaloid that acts as a potent uterotonic agent, used to control excessive uterine bleeding, such as postpartum hemorrhage. It primarily exerts its effects by stimulating the smooth muscle of the uterus, leading to sustained contraction and eventual cessation of bleeding.

**Why the Correct Answer is Right**
Ergometrine's mechanism of action is primarily attributed to its agonist activity at serotonin (5-HT2A) receptors in the uterine smooth muscle. This binding causes an increase in intracellular calcium, leading to muscle contraction and sustained uterine tone. Ergometrine's effects are also mediated through its alpha-adrenergic receptor agonist activity, which further contributes to vasoconstriction and uterine contraction.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as it does not accurately describe the primary mechanism of ergometrine in controlling postpartum hemorrhage.
**Option B:** This option is incorrect because it is not the primary mechanism of ergometrine's effect on the uterus.
**Option C:** This option is incorrect as it is not directly related to the mechanism by which ergometrine controls postpartum hemorrhage.

**Clinical Pearl / High-Yield Fact**
Ergometrine should be used with caution in patients with hypertension, as it can cause significant vasoconstriction and exacerbate hypervolemia. Additionally, it is essential to monitor patients closely for signs of ergotism, a condition characterized by vasoconstriction and gangrene of the extremities.

**Correct Answer: C. Stimulation of serotonin (5-HT2A) receptors in the uterine smooth muscle.**