**Core Concept**
Chromosomal abnormalities are the leading cause of fetal loss, particularly in the first trimester, due to the high incidence of aneuploidy in human conceptions.

**Why the Correct Answer is Right**
The most common chromosomal abnormality leading to fetal loss is trisomy 16, followed by trisomy 13 and trisomy 18. This is because meiotic nondisjunction occurs frequently during oogenesis, resulting in aneuploid oocytes. When fertilized by a normal sperm, these aneuploid oocytes can lead to trisomic conceptuses, which often undergo spontaneous abortion.

**Why Each Wrong Option is Incorrect**
**Option A:**
Congenital anomalies, such as cardiac defects or neural tube defects, are not the most common cause of fetal loss, although they do contribute significantly to stillbirths.

**Option B:**
Infections, such as rubella or cytomegalovirus, can cause fetal loss, but they are less common than chromosomal abnormalities.

**Option C:**
Immunological factors, such as isoimmunization, can contribute to fetal loss, particularly in the setting of Rh incompatibility. However, they are not the most common cause of fetal loss.

**Clinical Pearl / High-Yield Fact**
Chromosomal abnormalities are responsible for approximately 50% of first-trimester miscarriages, highlighting the importance of genetic counseling in women experiencing recurrent pregnancy loss.

**Correct Answer: D. Chromosomal abnormalities are the most common cause of fetal loss.**