**Core Concept**
The question is inquiring about the genetic factor involved in susceptibility to Neisseria infections, specifically Neisseria meningitidis and Neisseria gonorrhoeae. These bacteria cause severe and potentially life-threatening diseases, such as meningitis and septic arthritis. The underlying concept here is the role of genetic determinants in modulating susceptibility to these infections.

**Why the Correct Answer is Right**
The correct answer is related to the complement system, a group of proteins that plays a crucial role in the innate immune response. The most significant genetic factor involved in susceptibility to Neisseria infections is the deficiency of the late-acting complement components, particularly C5-C9. These components are essential for the formation of the membrane attack complex (MAC), which lyses bacterial cells. Individuals with deficiencies in these components are at increased risk of developing recurrent Neisseria infections.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect as it does not relate to the genetic factor involved in susceptibility to Neisseria infections.
* **Option B:** This option is incorrect as it is not a specific genetic factor related to susceptibility to Neisseria infections.
* **Option D:** This option is incorrect as it is not a genetic factor involved in the complement system or the innate immune response.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that individuals with deficiencies in the late-acting complement components are at increased risk of developing recurrent Neisseria infections. This knowledge is crucial for identifying patients who may benefit from prophylactic measures, such as antibiotic treatment or vaccination.

**Correct Answer:** C.