**Core Concept**
The most common cause of spontaneous abortion, also known as miscarriage, is a complex interplay of various factors. However, chromosomal abnormalities in the fetus are the primary reason for early pregnancy loss. These abnormalities can occur due to errors during meiosis or mitosis, leading to aneuploidy.

**Why the Correct Answer is Right**
Chromosomal abnormalities are responsible for approximately 50-60% of spontaneous abortions. This is because the meiotic process in oogenesis is more prone to errors, resulting in aneuploid gametes. Additionally, mosaicism, where a mixture of normal and abnormal cells is present, can also contribute to miscarriage. The most common chromosomal abnormalities associated with miscarriage include trisomy 16, trisomy 13, and trisomy 18.

**Why Each Wrong Option is Incorrect**
**Option B:** Uterine malformations, such as unicornuate or bicornuate uterus, can increase the risk of miscarriage, but they are not the most common cause. Uterine anomalies account for only a small percentage of spontaneous abortions.
**Option C:** Immunological factors, such as antiphospholipid syndrome or lupus anticoagulant, can contribute to recurrent pregnancy loss, but they are not the primary cause of spontaneous abortion.
**Option D:** Infections, such as pelvic inflammatory disease or endometritis, can increase the risk of miscarriage, but they are not the most common cause of spontaneous abortion.

**Clinical Pearl / High-Yield Fact**
A significant proportion of spontaneous abortions occur before the woman is even aware of her pregnancy, highlighting the importance of early detection and monitoring in high-risk pregnancies.

**✓ Correct Answer: A. Chromosomal abnormality**