**Core Concept**
Postpartum hemorrhage (PPH) is a leading cause of maternal morbidity and mortality worldwide. It is defined as the loss of more than 500 mL of blood after delivery. The causes of PPH can be broadly classified into four categories: uterine atony, lacerations, retained placental tissue, and coagulopathy.

**Why the Correct Answer is Right**
Uterine atony is the most common cause of postpartum hemorrhage, accounting for approximately 80% of cases. This occurs when the uterus fails to contract down effectively after delivery, leading to excessive bleeding. The uterine muscle is responsible for contracting and shrinking the uterus back to its pre-pregnant size. This process is mediated by the release of oxytocin, a hormone that stimulates uterine contractions.

**Why Each Wrong Option is Incorrect**
* **Option A:** Retained placental tissue is a common cause of PPH, but it is less common than uterine atony. Retained placental tissue can cause bleeding due to the incomplete separation of the placenta from the uterus.
* **Option B:** Coagulopathy is a rare cause of PPH, usually seen in cases of severe liver disease or consumption coagulopathy. Coagulopathy can lead to excessive bleeding due to the inability of the blood to clot properly.
* **Option C:** Lacerations, including vaginal and cervical lacerations, can cause PPH. However, they are less common than uterine atony and are often associated with other complications, such as infection.

**Clinical Pearl / High-Yield Fact**
The "4 Ts" mnemonic can be used to remember the causes of postpartum hemorrhage: Tone (uterine atony), Tissue (retained placental tissue), Trauma (lacerations), and Thrombin (coagulopathy).

**Correct Answer:** A. Uterine atony is the most common cause of postpartum hemorrhage, accounting for approximately 80% of cases.