**Core Concept**
Down syndrome, also known as trisomy 21, is a chromosomal anomaly characterized by an extra copy of chromosome 21. It is a classic example of an autosomal trisomy, where an individual has three copies of a particular autosome instead of the usual two.

**Why the Correct Answer is Right**
Trisomy 21 was the first chromosomal anomaly to be described. The discovery is attributed to French physician Jérôme Lejeune, who identified the condition in 1959. Lejeune's work laid the foundation for the understanding of chromosomal abnormalities and their relationship to various medical conditions.

**Why Each Wrong Option is Incorrect**
**Option A:** Turner syndrome is a chromosomal anomaly characterized by the absence of one X chromosome (45,X). While it is an important condition, it was not the first chromosomal anomaly to be described.

**Option B:** Klinefelter syndrome is a chromosomal anomaly characterized by an extra X chromosome in males (47,XXY). It was first described in 1942, after Lejeune's discovery of trisomy 21.

**Option C:** Patau syndrome is a chromosomal anomaly characterized by an extra copy of chromosome 13 (trisomy 13). It was first described in 1960, after Lejeune's discovery of trisomy 21.

**Option D:** Edward syndrome is a chromosomal anomaly characterized by an extra copy of chromosome 18 (trisomy 18). It was first described in 1960, after Lejeune's discovery of trisomy 21.

**Clinical Pearl / High-Yield Fact**
Lejeune's discovery of trisomy 21 marked the beginning of a new era in the understanding of chromosomal abnormalities. It also highlighted the importance of genetic testing in the diagnosis and management of various medical conditions.

**Correct Answer: C. Patau syndrome is a chromosomal anomaly characterized by an extra copy of chromosome 13 (trisomy 13).**