**Core Concept:** Cytogenetics is the study of chromosomes and their abnormalities, which can lead to various genetic disorders. Common chromosomal defects involve the number of chromosomes or their structure.

**Why the Correct Answer is Right:** The most common chromosomal defect is Down syndrome (Trisomy 21), which occurs when there is an extra copy of chromosome 21. This results in the overexpression of genes on this chromosome, leading to the characteristic features of Down syndrome, including intellectual disability, facial abnormalities, and heart defects.

**Why Each Wrong Option is Incorrect:**
A. Turner syndrome (Monosomy X) is a less common chromosomal defect, occurring when an individual has a missing X chromosome. This is not the most common chromosomal defect.
B. Klinefelter syndrome (Disomy X) is a less common chromosomal defect, occurring when an individual has an extra X chromosome. This is not the most common chromosomal defect.
C. DiGeorge syndrome is a chromosomal defect involving deletions on chromosome 22, not the most common chromosomal defect.
D. Uniparental disomy is a rare chromosomal defect where a single parent passes on two copies of a chromosome instead of one, but is not the most common chromosomal defect.

**Clinical Pearl:** Understanding the most common chromosomal defects can help guide clinical suspicion for specific syndromes and improve diagnostic accuracy in patients with developmental delays or congenital anomalies.

**Correct Answer:** D. Down syndrome (Trisomy 21) is the most common chromosomal defect.