**Core Concept**
Uterine atony is the leading cause of postpartum hemorrhage (PPH), resulting from inadequate uterine contractions due to loss of myometrial tone. Effective management requires rapid, reliable, and potent stimulation of uterine contractions to control bleeding.

**Why the Correct Answer is Right**
IV oxytocin is the first-line treatment for uterine atony in postpartum hemorrhage. It acts on oxytocin receptors in the myometrium, promoting strong, sustained contractions. A dose of 20 units IV is effective, safe, and rapidly available in the delivery room. It has a quick onset (within minutes), is well-tolerated, and is superior to oral agents in acute settings. The IV route ensures reliable absorption and immediate action, especially in a patient with active bleeding.

**Why Each Wrong Option is Incorrect**
Option A: Ergonovine is contraindicated in primigravidae and in patients with hypertension, which is a risk in pre-eclampsia. It also has a narrow safety margin and is not recommended in first-time deliveries.
Option C: Oral oxytocin is ineffective in acute bleeding due to delayed absorption and poor bioavailability. It is not suitable in a setting of active hemorrhage requiring immediate action.
Option D: Prostaglandin F2-alpha (e.g., misoprostol) is effective but requires higher doses and has more side effects (e.g., uterine hyperstimulation). It is also slower in onset and less reliable than IV oxytocin in acute PPH.

**Clinical Pearl / High-Yield Fact**
In PPH due to uterine atony, IV oxytocin is the first-line agent; it is faster, more reliable, and safer than alternatives in the immediate postpartum period. Always start with IV oxytocin before escalating to other agents.

✓ Correct Answer: B. 20 units of IV oxytocin