**Core Concept:** Complement system is a part of the adaptive immune system that plays a crucial role in the clearance of pathogens, immune complex deposition, and regulation of inflammation. It consists of a series of proteins that can be activated through three different pathways (classical, alternative, and lectin) and forms a cascade leading to the formation of an enzyme complex, C3 Convertase. This complex further activates C5, leading to the generation of the membrane attack complex (MAC), which directly lyses pathogens, and C5a, a potent chemoattractant and inflammatory mediator.

**Why the Correct Answer is Right:** Complement deficiencies are inherited disorders characterized by a reduced or absent complement proteins. Patients with deficiencies in C5 through C9 proteins (C5, C6, C7, C8, and C9) are particularly vulnerable to infections due to their inability to form the membrane attack complex (MAC), which is essential for directly lysing pathogens and preventing their entry into host tissues.

**Why Each Wrong Option is Incorrect:**

A. While some complement deficiencies can lead to increased susceptibility to infections, the correct answer does not specify a particular infection type; it refers to deficiencies in C5 through C9 proteins.

B. This option refers to infections caused by encapsulated bacteria, which is not specific to complement deficiencies affecting C5 through C9 proteins.

C. This option highlights a specific infection (Neisseria meningitides) but does not address the broader issue of complement deficiency affecting C5 through C9 proteins.

D. This option is not specific to complement deficiencies affecting C5 through C9 proteins and does not address the broader issue of the absence of MAC's lytic function.

**Clinical Pearl:** Complement deficiencies, particularly affecting C5 through C9 proteins, are associated with increased susceptibility to infections caused by encapsulated bacteria (e.g., Streptococcus pneumoniae, Haemophilus influenzae), mycobacteria, and encapsulated fungi. These deficiencies should be considered in the differential diagnosis of recurrent infections, particularly in patients with family history of similar deficiencies.

**Correct Answer: B. Infections caused by encapsulated bacteria.**

The correct answer is related to infections caused by bacteria, which are particularly susceptible to complement-mediated lysis due to their capsular polysaccharides. Infections with encapsulated bacteria, such as Streptococcus pneumoniae and Haemophilus influenzae, are more likely in patients with deficiencies affecting C5 through C9 proteins, as these proteins are essential for the formation of the membrane attack complex (MAC) that lyses these bacteria directly and prevents their entry into host tissues.