**Core Concept:**
The question is testing the knowledge about the complement system and its role in immune defense against Neisseria meningitidis, a bacterium that can cause meningitis and sepsis. The complement system is part of the innate immune response and helps to eliminate pathogens by direct bactericidal activity, opsonization, and chemotaxis. Components of the complement system include various proteins that can be activated through different pathways, such as the classical pathway, the lectin pathway, and the alternative pathway.

**Why the Correct Answer is Right:**
The correct answer, **D** (deficiency of C3), is related to the alternative pathway of the complement system. Complement component C3 is a crucial player in this pathway, and its deficiency leads to impaired activation of the alternative pathway. This, in turn, results in reduced bactericidal activity against Neisseria meningitidis and increased susceptibility to meningococcal infections.

**Why Each Wrong Option is Incorrect:**

**A:**
The question does not specify the deficiency of C4, but it is essential to mention that deficiency of C4 is also associated with increased susceptibility to Neisseria meningitidis infections, as C4 is a crucial component of the classical pathway and plays a role in the alternative pathway activation. However, C3 deficiency is more directly involved in the alternative pathway, making it the correct answer.

**B:**
Deficiency of C5 is incorrect because C5 is a terminal component of the classical and lectin pathways, and its deficiency does not directly affect the alternative pathway activation.

**C:**
Deficiency of C6 is incorrect because C6 is a component of the terminal pathway, which is separate from the alternative pathway. While deficiencies in terminal pathway components can lead to increased susceptibility to infections, C6 deficiency does not directly impact the alternative pathway activation.

**Clinical Pearl / High-Yield Fact:**
The complement system plays a crucial role in immune defense against Neisseria meningitidis and other pathogens. It is essential to understand the different pathways (classical, lectin, and alternative) and their components to differentiate between them and identify the correct answer, like deficiency of C3, in this case. Understanding these components and pathways can aid in diagnosing and treating patients with infections related to complement deficiencies.