**Core Concept**
Aneuploidy refers to the presence of an abnormal number of chromosomes in a cell, which can occur in the reproductive cells (gametes) or in the developing embryo. This genetic abnormality is a leading cause of miscarriages, failed in vitro fertilization (IVF), and birth defects.

**Why the Correct Answer is Right**
Aneuploidy can arise due to errors during meiosis, the process by which reproductive cells are formed. These errors can result in an abnormal number of chromosomes, leading to aneuploidy. The most common aneuploidies in humans include trisomy 16, trisomy 13, and trisomy 21 (Down syndrome). Aneuploidy can also occur in somatic cells, contributing to various diseases, including cancer.

**Why Each Wrong Option is Incorrect**
**Option A:** Aneuploidy is indeed the most common genetic abnormality, accounting for up to 50% of clinical miscarriages. This statement is true.
**Option B:** Aneuploidy can occur in both autosomal and sex chromosomes, making this statement true. Aneuploidy can involve an extra or missing sex chromosome, leading to conditions such as Turner syndrome (45,X) or Klinefelter syndrome (47,XXY).
**Option C:** Aneuploidy is a leading cause of miscarriages, failed IVF, and birth defects, making this statement true. The presence of aneuploidy in a developing embryo can lead to miscarriage or failed implantation.

**Clinical Pearl / High-Yield Fact**
A key aspect of aneuploidy is that it can occur in both reproductive cells and somatic cells. Understanding the mechanisms of aneuploidy is crucial for diagnosing and managing genetic disorders.

**Correct Answer: D.**