## **Core Concept**
Complement-mediated lysis is a process by which the immune system eliminates pathogens through the formation of a membrane attack complex (MAC). This process involves the activation of the complement system, a cascade of protein interactions that results in the insertion of a pore into the target cell membrane, leading to cell lysis. The key component responsible for this lytic activity is a complex of complement proteins.

## **Why the Correct Answer is Right**
The correct answer, **Membrane Attack Complex (MAC)**, is the terminal product of the complement cascade that directly causes cell lysis. MAC is formed through the sequential assembly of complement components C5b, C6, C7, C8, and multiple molecules of C9. This complex inserts into the lipid bilayer of the target cell membrane, forming a pore that disrupts membrane integrity, leading to an influx of ions and water, and ultimately, cell swelling and bursting.

## **Why Each Wrong Option is Incorrect**
- **Option A: C1q** - This is the first component of the classical pathway of complement activation, recognizing and binding to antibodies or other ligands on the surface of pathogens. It does not directly mediate lysis.
- **Option B: C3b** - While C3b is a crucial fragment in the complement cascade, acting as an opsonin and helping to amplify the complement response, it does not form the pore responsible for cell lysis.
- **Option D: Factor B** - This is a component of the alternative pathway of complement activation, involved in the formation of the C3 convertase. It does not directly participate in the formation of the MAC.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that deficiencies in late complement components (C5-C9) can lead to increased susceptibility to infections, particularly with *Neisseria* species, due to impaired ability to form the MAC and lyse these pathogens.

## **Correct Answer: C. Membrane Attack Complex.**