**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 anomalies. It is a microdeletion syndrome resulting from a deletion of a small segment of chromosome 22.

**Why the Correct Answer is Right**
The correct answer is D. 22 because De-George syndrome is caused by a microdeletion involving the long arm of chromosome 22, specifically the q11.2 region. This deletion leads to the absence or underdevelopment of the thymus and parathyroid glands, resulting in immunodeficiency and hypocalcemia. The deletion also affects the development of the third and fourth pharyngeal pouches, leading to cardiac anomalies and other congenital defects. The 22q11.2 region is a critical area for the development of these structures, and its deletion results in the characteristic features of De-George syndrome.

**Why Each Wrong Option is Incorrect**
**Option A:** Chromosome 7 is associated with Williams syndrome, a different microdeletion syndrome characterized by distinct clinical features such as hypercalcemia, supravalvular aortic stenosis, and a friendly, outgoing personality.

**Option B:** Chromosome 15 is associated with Prader-Willi syndrome and Angelman syndrome, two distinct genetic disorders resulting from different types of mutations or deletions in the 15q11-q13 region.

**Option C:** Chromosome 17 is associated with several genetic disorders, including Smith-Magenis syndrome and Wiedemann-Rautenstrauch syndrome, but not De-George syndrome.

**Clinical Pearl / High-Yield Fact**
De-George syndrome is often associated with cleft palate, cardiac defects, and immunodeficiency, making it a critical condition to recognize in pediatric patients with congenital anomalies.

**✓ Correct Answer: D. 22**