## **Core Concept**
The question pertains to the genetic factors that predispose individuals to infections caused by *Neisseria* species, such as *Neisseria meningitidis* (meningococcus) and *Neisseria gonorrhoeae* (gonococcus). The focus is on identifying the most common genetic factor that increases susceptibility to these infections. This involves understanding the role of the complement system and specific genetic deficiencies in host defense against encapsulated bacteria.

## **Why the Correct Answer is Right**
The correct answer, **C. C7-C9 deficiency**, highlights a crucial aspect of innate immunity. The complement system, particularly the terminal components (C5-C9), plays a vital role in defending against *Neisseria* infections by forming the membrane attack complex (MAC), which disrupts bacterial cell membranes, leading to their lysis and death. Deficiencies in the terminal complement components (C5-C9) are well-documented risk factors for recurrent *Neisseria* infections, especially *Neisseria meningitidis*. This is because *Neisseria* species are particularly susceptible to complement-mediated lysis.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While properdin deficiency does increase susceptibility to *Neisseria* infections, particularly *Neisseria meningitidis*, it is less common compared to terminal complement component deficiencies. Properdin is involved in the alternative pathway of complement activation.
- **Option B:** C3 deficiency is a significant immunodeficiency that affects both the classical and alternative pathways of complement activation. However, it is associated with a broader range of infections, not just *Neisseria*, and is less specifically linked to *Neisseria* infections compared to terminal complement deficiencies.
- **Option D:** While other complement deficiencies can increase the risk of infections, **C7-C9 deficiency** is notably associated with increased susceptibility to recurrent *Neisseria* infections, making the other options less specific or less common causes.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that deficiencies in the terminal complement components (C5-C9) are associated with an increased risk of recurrent *Neisseria* infections. This knowledge is crucial for managing patients with recurrent *Neisseria* infections, as it may guide further investigations and management strategies.

## **Correct Answer: C. C7-C9 deficiency**