## **Core Concept**
The question tests the understanding of the relationship between complement system deficiencies and susceptibility to specific infections, notably those caused by Neisseria species. The complement system is a part of the innate immune system that enhances (complements) the ability of antibodies and phagocytic cells to clear microbes and damaged cells from an organism. Neisseria infections, such as Neisseria meningitidis (meningococcus) and Neisseria gonorrhoeae, are particularly associated with deficiencies in certain components of the complement system.

## **Why the Correct Answer is Right**
The correct answer, **C. Late complement components (C5-C9)**, is right because deficiencies in the late complement components (C5-C9) significantly increase susceptibility to recurrent Neisseria infections, particularly Neisseria meningitidis. The late complement components are crucial for the formation of the membrane attack complex (MAC), which creates pores in the bacterial cell wall, leading to bacterial lysis and death. Without these components, the body is less capable of eliminating Neisseria species, leading to an increased risk of infections.

## **Why Each Wrong Option is Incorrect**
- **Option A:** Early complement components (C1-C4) are involved in the classical pathway of complement activation and are more commonly associated with autoimmune diseases, such as systemic lupus erythematosus (SLE), rather than increased susceptibility to Neisseria infections.
- **Option B:** Alternative complement pathway (Factor B, Factor D) deficiencies can also increase susceptibility to infections, but they are not as specifically linked to Neisseria infections as deficiencies in the late complement components.
- **Option D:** Mannose-binding lectin (MBL) is part of the innate immune system and plays a role in the recognition of pathogens, but its deficiency is more commonly associated with an increased risk of infections in general, rather than specifically with Neisseria infections.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that patients with deficiencies in late complement components (C5-C9) are particularly susceptible to recurrent Neisseria infections, and this association can be a critical clue in the diagnosis of complement deficiencies. Prophylactic measures, such as vaccination against Neisseria meningitidis, are often recommended for individuals with known complement deficiencies.

## **Correct Answer:** C. Late complement components (C5-C9).