**Core Concept**
The question pertains to the concept of aneuploidy, specifically focusing on the types of chromosomal abnormalities that are multiples of the haploid number of chromosomes, which is 23 in humans. Aneuploidy arises from errors during cell division, resulting in an abnormal number of chromosomes in an individual's cells.

**Why the Correct Answer is Right**
Monosomy, the correct answer, is a type of aneuploidy where one chromosome is missing. This occurs when a pair of homologous chromosomes fails to separate during meiosis, resulting in one individual inheriting only one copy of a particular chromosome. Monosomy is often incompatible with life, and examples include Turner syndrome (45,X), where one X chromosome is missing. Monosomy is an exact multiple of the haploid number, as it involves the loss of a single chromosome.

**Why Each Wrong Option is Incorrect**
**Option A:** Polyploidy is a type of aneuploidy where an individual has more than two complete sets of chromosomes. This is not an exact multiple of the haploid number, as it involves the gain of multiple sets of chromosomes.
**Option B:** Trisomy is a type of aneuploidy where an individual has an extra copy of a particular chromosome. This is not an exact multiple of the haploid number, as it involves the gain of a single extra chromosome.
**Option C:** Tetraploidy is a type of aneuploidy where an individual has four sets of chromosomes. This is not an exact multiple of the haploid number, as it involves the gain of multiple sets of chromosomes.

**Clinical Pearl / High-Yield Fact**
Aneuploidy is a significant cause of genetic disorders, and understanding the different types of aneuploidy is crucial for diagnosing and managing genetic conditions. Remember that monosomy is a critical type of aneuploidy that often results in severe developmental abnormalities.

**Correct Answer:** A. Monosomy