**Core Concept**
Postpartum endometritis is a severe infection of the endometrium, which occurs after childbirth. It is a common cause of maternal morbidity and mortality worldwide. The underlying cause is typically an ascending infection from the vagina and cervix.

**Why the Correct Answer is Right**
The most common cause of postpartum endometritis is the transfer of bacteria from the vagina and cervix to the endometrium, facilitated by the presence of bacteria in the birth canal. This is often due to an overgrowth of normal vaginal flora, such as _Escherichia coli_ and Group B Streptococcus (GBS), which can ascend to the upper genital tract during labor or postpartum. The endometrium is particularly susceptible to infection due to its rich blood supply and the presence of a foreign body, such as the placental tissue.

**Why Each Wrong Option is Incorrect**
* **Option A:** While _Staphylococcus aureus_ can cause endometritis, it is not the most common cause.
* **Option B:** _Clostridium difficile_ can cause postpartum infections, but it is not the primary cause of endometritis.
* **Option C:** _Candida albicans_ can cause vaginal yeast infections, but it is not typically associated with postpartum endometritis.

**Clinical Pearl / High-Yield Fact**
To prevent postpartum endometritis, it is essential to maintain good vaginal hygiene during labor and postpartum, and to administer antibiotics prophylactically to high-risk patients.

**Correct Answer:** D.