**Core Concept**
Postpartum hemorrhage (PPH) is a leading cause of maternal mortality worldwide. It is defined as the loss of more than 500 mL of blood after delivery. In spontaneous deliveries, PPH can arise due to various factors, including uterine atony, retained placental tissue, and lacerations.

**Why the Correct Answer is Right**
The correct answer is uterine atony. Uterine atony occurs when the uterus fails to contract effectively after delivery, leading to excessive bleeding. This is often due to a combination of factors, including prolonged labor, multiple gestations, and the use of oxytocin or other uterotonic agents. The uterus requires adequate blood supply, optimal uterine tone, and sufficient levels of oxytocin to contract effectively. In the absence of these factors, uterine atony can occur.

**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 spontaneous deliveries.
* **Option B:** Lacerations can cause PPH, but they are relatively rare compared to uterine atony.
* **Option D:** Coagulopathy is a rare cause of PPH and is not the most common cause in spontaneous deliveries.

**Clinical Pearl / High-Yield Fact**
Uterine atony is the most common cause of PPH in spontaneous deliveries. It is essential to identify and manage uterine atony promptly to prevent excessive bleeding.

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