## Core Concept
Chromosomal trisomies refer to a type of aneuploidy where a person has an extra chromosome, making a total of three copies of that chromosome instead of the usual two. This condition can lead to various genetic disorders, depending on which chromosome is affected.

## Why the Correct Answer is Right
Trisomy 21, also known as Down syndrome, is a well-known chromosomal disorder. Similarly, Trisomy 18 (Edwards syndrome) and Trisomy 13 (Patau syndrome) are also recognized trisomies. However, Turner syndrome is characterized by the presence of only one X chromosome (45,X), not a trisomy. Therefore, Turner syndrome does not fit with the others as a trisomy.

## Why Each Wrong Option is Incorrect
* **Option A:** Trisomy 21 - This is indeed a chromosomal trisomy, leading to Down syndrome.
* **Option B:** Trisomy 18 - This is also a chromosomal trisomy, known as Edwards syndrome.
* **Option C:** Trisomy 13 - This too is a chromosomal trisomy, referred to as Patau syndrome.

## Clinical Pearl / High-Yield Fact
A key point to remember is that while Down syndrome (Trisomy 21), Edwards syndrome (Trisomy 18), and Patau syndrome (Trisomy 13) are all trisomies, Turner syndrome is not; it's a monosomy. This distinction is crucial for understanding the genetic basis of these disorders.

**Correct Answer: D. Turner syndrome.**