**Core Concept**
Aneuploidy refers to the presence of an abnormal number of chromosomes in a cell, which is a common feature of various genetic disorders and cancers. This phenomenon occurs due to errors during meiosis or mitosis, leading to an imbalance in the genetic material.

**Why the Correct Answer is Right**
Aneuploidy typically arises from nondisjunction, which is the failure of homologous chromosomes to separate properly during meiosis I. This can occur due to errors in the spindle assembly checkpoint or the presence of mutations in genes involved in this process, such as _BUB1_ or _BUBR1_. Nondisjunction can also occur during mitosis, often as a result of errors in the centrosome duplication cycle.

**Why Each Wrong Option is Incorrect**
**Option A:** Mitosis is the process of cell division that results in two daughter cells with the same number of chromosomes as the parent cell, so it cannot cause aneuploidy.
**Option B:** While genetic mutations can contribute to aneuploidy, they are not the primary cause of this phenomenon.
**Option C:** Chromosomal breakage and rejoining, also known as chromosomal rearrangement, can lead to genetic abnormalities, but it is not the primary mechanism underlying aneuploidy.

**Clinical Pearl / High-Yield Fact**
Aneuploidy is a hallmark of many types of cancer, including Down syndrome, Turner syndrome, and Klinefelter syndrome, which are all caused by chromosomal abnormalities.

**Correct Answer: C.**