**Core Concept**
Severe Combined Immunodeficiency (SCID) is a group of rare, life-threatening disorders caused by mutations in genes involved in the development and function of the immune system. These disorders result in impaired T cell and/or B cell development, leading to severe recurrent infections and potentially life-threatening complications.

**Why the Correct Answer is Right**
The correct answer is related to the enzyme adenosine deaminase (ADA). SCID caused by ADA deficiency is one of the most common forms of SCID. ADA is responsible for the breakdown of toxic compounds, such as deoxyadenosine, which accumulate in the absence of functional ADA. This accumulation leads to the depletion of T cells and B cells, resulting in impaired immune function. ADA deficiency is usually inherited in an autosomal recessive pattern, making it a significant cause of SCID.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it does not specify a known cause of SCID. While there are many genes involved in the development and function of the immune system, ADA deficiency is a well-established cause of SCID.
* **Option B:** This option is incorrect because it references a different condition. Chronic granulomatous disease (CGD) is a disorder of phagocytic cells, but it is not a cause of SCID.
* **Option C:** This option is incorrect because it is too broad. While T cell deficiency can be a cause of SCID, this option does not specify a particular genetic cause.

**Clinical Pearl / High-Yield Fact**
SCID is often referred to as the "bubble boy disease" due to the severe immunocompromised state of affected individuals, which requires them to live in a sterile environment to prevent infections.

**Correct Answer: C. Adenosine deaminase (ADA) deficiency**