Defective chromosome associated with De-George syndrome is?

**Core Concept**
De-George syndrome, also known as 22q11.2 deletion syndrome, is a congenital disorder characterized by thymic hypoplasia, parathyroid gland aplasia, and cardiac defects. The condition arises due to a microdeletion in a region of chromosome 22, specifically the long arm (q) at band 11.2.

**Why the Correct Answer is Right**
The deletion of chromosome 22q11.2 disrupts the development of the third and fourth pharyngeal pouches, leading to thymic hypoplasia and parathyroid gland aplasia. This region also contains genes critical for cardiac development, contributing to the cardiac anomalies observed in patients with De-George syndrome. The deletion affects multiple genes, including TBX1, which plays a crucial role in the development of the pharyngeal arches and pouches.

**Why Each Wrong Option is Incorrect**
* **Option B:** Chromosome 18q deletion is associated with Edwards syndrome, a different congenital disorder characterized by intellectual disability, heart defects, and various other physical abnormalities.
* **Option C:** Chromosome 21 trisomy is associated with Down syndrome, a condition that presents with intellectual disability, characteristic facial features, and an increased risk of certain medical conditions.
* **Option D:** Chromosome 5p deletion is associated with Cri-du-chat syndrome, a rare genetic disorder characterized by a high-pitched cry in infancy and various developmental delays.

**Clinical Pearl / High-Yield Fact**
The 22q11.2 deletion syndrome is often referred to as "velocardiofacial syndrome" due to the presence of cleft palate, cardiac defects, and characteristic facial features. Patients with this condition are at an increased risk of developing psychiatric disorders, including schizophrenia and anxiety disorders.

**Correct Answer:** A. 22q11.2 deletion.