The Question is from neet-zoology , and it was created by the author BALASOURY

Question

A novel immunodeficiency is characterized by recurrent bacterial infections, particularly with encapsulated bacteria.  Serum analysis reveals normal levels of IgG, IgM, and IgA, but significantly reduced levels of complement component C3. Which pathway of complement activation is MOST likely primarily affected?

BALASOURY · Accepted Answer

## CORE INFORMATION
✓ **Answer**: C
## CONCEPT FRAMEWORK
	**Primary Concept**: Complement Activation Pathways
	**Related Topics**: Innate Immunity, Antibody Function, Bacterial Infections
	**Prerequisites**: Understanding of the three complement pathways (classical, lectin, alternative) and their roles in immunity.
## SOLUTION PATHWAY
**Given Information**:
	Recurrent bacterial infections (especially encapsulated bacteria).
	Normal IgG, IgM, and IgA levels.
	Significantly reduced C3 levels.
**Method & Steps**:
	1. **Consider the role of antibodies**: Normal antibody levels suggest the classical pathway (activated by antigen-antibody complexes) is likely functional.
		Reasoning: The classical pathway is initiated by antibody binding.  If antibodies were deficient, we'd expect issues with this pathway.
	2. **Focus on C3**:  C3 is a central component of all three complement pathways.  Its reduction points to a pathway that continuously generates C3.
		Working:  The alternative pathway constantly generates small amounts of C3 through a 'tick-over' mechanism. This is crucial for rapid response to pathogens, especially encapsulated bacteria.
		Key Point: C3 deficiency, despite normal antibody levels, strongly suggests a problem with C3 production or consumption via the alternative pathway.
	3. **The importance of the alternative pathway for encapsulated bacteria**: The alternative pathway is particularly important for opsonizing encapsulated bacteria, which are resistant to phagocytosis without complement.
		Result: The alternative pathway is the most likely affected.
		Verification: Reduced C3, despite normal antibody levels and susceptibility to encapsulated bacteria, aligns with a dysfunctional alternative pathway.
## OPTION ANALYSIS
[A] :x: Classical pathway:  Normal IgG, IgM, and IgA levels suggest this pathway is likely intact.  The classical pathway is initiated by antibody binding, and normal antibody levels imply functionality.
[B] :x: Lectin pathway: While the lectin pathway also contributes to innate immunity, it is not as crucial for clearing encapsulated bacteria as the alternative pathway.  Also, the question doesn't indicate any issues with mannose-binding lectin (MBL), which initiates this pathway.
[C] ✓: Alternative pathway: This pathway is constantly active and is critical for opsonizing encapsulated bacteria. Reduced C3, even with normal antibody levels, strongly suggests a defect in this pathway.
[D] :x: Terminal pathway: The terminal pathway is the final common pathway for all three complement pathways, leading to the membrane attack complex (MAC). While C3 is involved upstream, a primary defect in the terminal pathway alone would not explain the specific susceptibility to encapsulated bacteria.
## LEARNING AIDS
	**Common Mistakes**:
		1. Focusing solely on antibodies: Remember that complement can be activated independent of antibodies.
		2. Overlooking the continuous nature of the alternative pathway: This pathway's 'tick-over' mechanism is key to understanding its role in C3 generation.
	**Quick Tips**:
		1. Encapsulated bacteria are particularly vulnerable to the alternative pathway.
		2. C3 is a central component of all complement pathways, but its continuous generation is a hallmark of the alternative pathway.

BALASOURY · Answer

Classical pathway

BALASOURY · Answer

Lectin pathway

BALASOURY · Answer

The correct Option is "Alternative pathway"

BALASOURY · Answer

Terminal pathway

NEET Zoology MCQs

Select Chapter

Question Tags

Question Tags

Question Tags

Question Tags

Question Tags

Question Tags

Question Tags

Question Tags

Question Tags

Question Tags

NEET Subjects

Our Bots

Company