**Core Concept**
Retained placenta, also known as retained placental tissue, refers to the failure of the placenta to separate from the uterus following delivery. This condition is a significant obstetric complication that can lead to hemorrhage, infection, and other maternal and fetal morbidities.

**Why the Correct Answer is Right**
The most common cause of retained placenta is uterine atony, which is the failure of the uterus to contract after delivery. This can be due to various factors, including excessive uterine distension during labor, prolonged labor, or the use of uterotonics. When the uterus fails to contract, it cannot effectively separate the placenta from the uterine wall, leading to retention. Additionally, manual removal of the placenta (MROP) is often required in such cases.

**Why Each Wrong Option is Incorrect**
**Option A:** Placenta accreta is a rare condition where the placenta invades the uterine wall, but it is not a common cause of retained placenta.
**Option B:** Uterine anomalies, such as a bicornuate uterus, can increase the risk of retained placenta, but they are not the primary cause.
**Option C:** Placental abruption, where the placenta separates from the uterus prematurely, is a different condition that can lead to fetal distress and maternal bleeding, but it is not a common cause of retained placenta.

**Clinical Pearl / High-Yield Fact**
Uterine atony is the most common cause of retained placenta, and prompt recognition and treatment are crucial to prevent maternal and fetal complications.

**Correct Answer:** D. Uterine atony