**Core Concept**
The question is testing the student's knowledge of the etiology of abortion, specifically the most common cause. Abortion refers to the termination of a pregnancy before the fetus is viable. The underlying principle being tested is the identification of the primary cause of abortion in a population.

**Why the Correct Answer is Right**
The most common cause of abortion is **miscarriage**, which is often due to chromosomal abnormalities in the fetus. This is because many fertilized eggs have genetic defects that make them non-viable, leading to spontaneous abortion. The process of miscarriage can be influenced by hormonal changes, uterine anomalies, and other factors, but chromosomal abnormalities are the primary cause.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because while infections can contribute to abortion, they are not the most common cause. Infections can cause endometritis and pelvic inflammatory disease, leading to pregnancy loss.
* **Option B:** This option is incorrect because while uterine anomalies can contribute to miscarriage, they are rare and not the most common cause. Uterine anomalies can lead to malformations of the uterus, cervix, or vagina, which can increase the risk of pregnancy loss.
* **Option C:** This option is incorrect because while maternal age can increase the risk of miscarriage, it is not the most common cause. Maternal age can increase the risk of chromosomal abnormalities, but it is not the primary cause of miscarriage.

**Clinical Pearl / High-Yield Fact**
It's essential to note that chromosomal abnormalities are the primary cause of miscarriage, and this is often due to errors in meiosis or mitosis during fetal development.

**Correct Answer:** D.