**Core Concept**
Down syndrome is a genetic disorder caused by an abnormality in the number of copies of chromosome 21. The condition is characterized by intellectual disability, delayed speech, and physical growth delays. The most common cause of Down syndrome is trisomy 21, where an individual has an extra copy of chromosome 21.

**Why the Correct Answer is Right**
A deleted 21 chromosome is not a recognized or documented cause of Down syndrome. In fact, Down syndrome is typically associated with an extra copy of chromosome 21, either due to trisomy 21 (where all cells have an extra chromosome 21), mosaicism (where some cells have an extra chromosome 21), or a Robertsonian translocation (where a part of chromosome 21 breaks off and attaches to another chromosome). A deleted 21 chromosome would result in a condition known as 21q deletion syndrome, which is a different genetic disorder.

**Why Each Wrong Option is Incorrect**
**Option B:** Trisomy 21 is the most common cause of Down syndrome, where an individual has an extra copy of chromosome 21. This results in intellectual disability, delayed speech, and physical growth delays.
**Option C:** A Robertsonian translocation, also known as a Robesonia translocation, is a type of chromosomal abnormality where a part of chromosome 21 breaks off and attaches to another chromosome. This can cause Down syndrome, especially if the individual inherits two copies of the translocated chromosome 21.
**Option D:** Mosaicism is a condition where some cells have an extra copy of chromosome 21, while others do not. This can occur in individuals with Down syndrome, especially those who are not overtly symptomatic.

**Clinical Pearl / High-Yield Fact**
Down syndrome is a genetic disorder caused by an abnormal number of copies of chromosome 21. The condition is characterized by intellectual disability, delayed speech, and physical growth delays. It's essential to recognize the different types of chromosomal abnormalities that can cause Down syndrome, including trisomy 21, mosaicism, and Robertsonian translocations.

✓ Correct Answer: A. Deleted 21