**Core Concept**
The III step of management of atonic uterus involves the administration of uterotonic agents to stimulate uterine contractions and prevent or treat postpartum hemorrhage. This step is crucial in maintaining uterine tone and controlling bleeding.

**Why the Correct Answer is Right**
Oxytocin is a potent uterotonic agent that stimulates uterine contractions by binding to oxytocin receptors on the smooth muscle cells of the uterus. This binding causes an increase in intracellular calcium, leading to muscle contraction. Oxytocin is often administered via intravenous infusion or intramuscular injection to treat atonic uterus and prevent postpartum hemorrhage.

**Why Each Wrong Option is Incorrect**
**Option A:** Methergine (Methylergonovine) is a potent ergot alkaloid that can cause severe vasoconstriction and is contraindicated in cases of hypertension or preeclampsia. It is not the preferred agent for atonic uterus.
**Option B:** Ergotamine is another ergot alkaloid that can cause vasoconstriction and is not typically used to treat atonic uterus.
**Option C:** Prostaglandin E2 (PGE2) can stimulate uterine contractions but is not the preferred agent for atonic uterus due to its potential to cause uterine hyperstimulation and other side effects.

**Clinical Pearl / High-Yield Fact**
Remember that oxytocin is the first-line uterotonic agent for atonic uterus, and its administration should be titrated carefully to avoid excessive uterine contractions.

**Correct Answer:** C.