Thymic hypoplasia is seen in –
**Core Concept**
Thymic hypoplasia refers to the underdevelopment of the thymus gland, a crucial organ responsible for the maturation and selection of T lymphocytes (T cells). The thymus is essential for the normal functioning of the immune system, and its underdevelopment can lead to severe immunodeficiency.
**Why the Correct Answer is Right**
Thymic hypoplasia is often seen in DiGeorge Syndrome, also known as 22q11.2 deletion syndrome. This genetic disorder results from a deletion of a small segment of chromosome 22, which leads to the underdevelopment of the thymus and parathyroid glands. The absence or underdevelopment of the thymus impairs the maturation of T cells, leading to severe immunodeficiency. The parathyroid glands are also affected, resulting in hypocalcemia due to decreased parathyroid hormone production.
**Why Each Wrong Option is Incorrect**
* **Option A:** Turner Syndrome is a genetic disorder that affects females, characterized by the partial or complete absence of one X chromosome. While it can lead to various health issues, it is not typically associated with thymic hypoplasia.
* **Option B:** Down Syndrome is a genetic disorder caused by an extra copy of chromosome 21. While it can lead to various health issues, including immune system abnormalities, it is not typically associated with thymic hypoplasia.
* **Option C:** Bloom Syndrome is a rare genetic disorder characterized by short stature, a small head, and a weakened immune system. However, it is not typically associated with thymic hypoplasia.
**Clinical Pearl / High-Yield Fact**
DiGeorge Syndrome is a classic example of the "two-hit hypothesis," where a genetic deletion (the first hit) leads to the underdevelopment of the thymus and parathyroid glands, and subsequent infections (the second hit) lead to the clinical manifestations of immunodeficiency.
**Correct Answer: C.