**Core Concept**
Postpartum hemorrhage (PPH) is a leading cause of maternal mortality worldwide, characterized by excessive bleeding following delivery. The most common cause of PPH is uterine atony, which is a failure of the uterus to contract effectively after delivery.

**Why the Correct Answer is Right**
Uterine atony occurs due to the exhaustion of uterine muscles, leading to inadequate contraction and subsequent bleeding. This is often caused by factors such as prolonged labor, multiple gestations, or the use of oxytocic agents, which can lead to uterine muscle fatigue. The uterus requires adequate blood supply, oxygenation, and hormonal support to contract effectively. Any disruption in these factors can lead to uterine atony.

**Why Each Wrong Option is Incorrect**
* **Option A:** Retained placental tissue is a possible cause of PPH but is not the most common cause. Retained placenta can lead to bleeding due to the continued placental circulation and the release of prostaglandins, which stimulate uterine contractions.
* **Option B:** Lacerations or episiotomies can cause PPH but are less common than uterine atony. These injuries can lead to bleeding from the genital tract, but they are typically easier to identify and manage than uterine atony.
* **Option C:** Uterine inversion is a rare cause of PPH, characterized by the uterus turning inside out. This condition requires immediate medical attention and is often associated with retained placental tissue.

**Clinical Pearl / High-Yield Fact**
The "4Ts" are a useful mnemonic to remember the causes of PPH: Tone (uterine atony), Tissue (retained placental tissue), Trauma (lacerations or episiotomies), and Thrombin (coagulopathy).

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