**Core Concept**
Postpartum hemorrhage (PPH) is a leading cause of maternal morbidity and mortality worldwide. In multiparous women, the risk of PPH is higher due to uterine atony, which is the most common cause of PPH.

**Why the Correct Answer is Right**
Uterine atony occurs when the uterus fails to contract down effectively after delivery, leading to excessive bleeding. This is often due to the uterus being overdistended during pregnancy, which can lead to fatigue of the uterine muscle. In multiparous women, the risk of uterine atony is higher due to the increased size and weight of the uterus with each subsequent pregnancy. Additionally, the uterus may not contract as effectively in multiparous women due to the accumulation of scar tissue from previous uterine surgeries or pregnancies.

**Why Each Wrong Option is Incorrect**
**Option A:** Retained placental tissue is a common cause of PPH, but it is not the most common cause in multiparous women.
**Option B:** Uterine rupture is a rare cause of PPH and is more commonly associated with previous uterine surgery or a history of cesarean delivery.
**Option C:** Coagulopathy is a rare cause of PPH and is more commonly associated with underlying bleeding disorders or anticoagulant therapy.

**Clinical Pearl / High-Yield Fact**
Uterine atony is often preceded by a "fourth-degree uterine atony," which is characterized by the uterus being extremely soft and boggy to the touch. This is a critical sign that should prompt immediate action to prevent PPH.

**Correct Answer: C. Uterine atony.**