**Question:** Factor present in the final common terminal complement pathway is -

A. C5a
B. C3b
C. C6
D. C7

**Core Concept:**
The complement system is part of the immune response that aids in the recognition, opsonization, and destruction of pathogens. It is a series of proteins that act together to eliminate the invading microorganisms. The final common terminal pathway involves the activation of C5, which leads to the formation of two main factors: C5a and C5b.

**Why the Correct Answer is Right:**
C5 is activated after the activation of the complement system. Once activated, C5 is cleaved into two factors: C5a (an anaphylatoxin) and C5b (which initiates the formation of the membrane attack complex). C5a is a potent inflammatory mediator that attracts neutrophils and other immune cells to the site of infection, while C5b plays a crucial role in the direct lysis of pathogens.

**Why Each Wrong Option is Incorrect:**
A. C5a: While C5a is an important mediator that attracts immune cells, it is cleaved from C5, not a separate factor.
B. C3b: C3b is a crucial factor in the classical and alternative pathways, but not the final common terminal pathway.
C. C6: C6 is a component of the membrane attack complex, formed downstream of C5b, not part of the final common terminal pathway.
D. C7: C7 is part of the membrane attack complex, formed downstream of C5b, not a factor in the final common terminal pathway.

**Clinical Pearl:**
Understanding the complement system and its pathways is essential for understanding the immune response and its role in host defense against pathogens. Knowledge of these pathways can help in interpreting clinical cases involving complement deficiencies or dysregulations, which may lead to increased susceptibility to infections or autoimmune disorders.