**Question:** N. meningitis can be due to deficiency of this complement system:

A. Complement system (classical pathway)
B. Complement system (lectin pathway)
C. Complement system (mannose-binding lectin pathway)
D. Complement system (alternative pathway)

**Correct Answer:** D. Complement system (alternative pathway)

**Core Concept:** The complement system is a crucial part of the immune response, involving a series of proteins that work together to neutralize pathogens, recruit immune cells, and increase vascular permeability. There are three main pathways by which complement activation can occur:

1. Classical pathway: Activated by antibodies (IgG, IgA, IgM) binding to the pathogen surface, leading to the formation of C4b and C3b.
2. Lectin pathway: Activated by mannose-binding lectin (MBL) binding to carbohydrate patterns on the pathogen surface, leading to the formation of C4b and C3b.
3. Alternative pathway: This pathway is constitutively active and involves the consumption of C3b and the formation of C3 convertase (C3bBb) when a pathogen is recognized.

**Why the Correct Answer is Right:**

N. meningitis, which is inflammation of the protective membranes covering the brain, can be caused by bacterial pathogens invading the central nervous system (CNS). In cases of deficiency of the complement system, specifically the alternative pathway (C3bBb), the immune response falls short in neutralizing and eliminating the invading pathogens, leading to increased susceptibility to infections, including N. meningitis.

**Why Other Options are Incorrect:**

A. Complement system (classical pathway) and B. Complement system (lectin pathway) deficiencies are not directly linked to meningitis caused by Neisseria meningitidis. These pathways primarily involve antibodies and MBL, respectively, and are not directly associated with meningitis caused by this bacterium.

C. Complement system (mannose-binding lectin pathway): Although the lectin pathway is involved in the immune response, deficiency in this pathway is not directly linked to meningitis caused by Neisseria meningitidis. The alternative pathway is more closely related to this bacterial infection.

**Clinical Pearls:**

A deficiency in the alternative complement pathway can lead to increased susceptibility to infections, as the complement system plays a crucial role in the immune response against bacteria and other pathogens. Infections, including meningitis, can occur when the immune system fails to neutralize and eliminate the invading pathogens effectively.

Understanding the differences between the complement system pathways is essential for recognizing potential immune deficiencies and their clinical implications. In this case, the deficiency in the alternative pathway is the correct answer as it is directly related to Neisseria meningitidis infection and its associated meningitis.