Which of the following factors are present in the final common terminal complement pathway?

**Core Concept**
The terminal complement pathway, also known as the membrane attack complex (MAC), is the final stage of the complement system's activation, leading to the formation of pores in the cell membrane and ultimately, cell lysis. This pathway is critical in eliminating pathogens from the body.

**Why the Correct Answer is Right**
The terminal complement pathway involves the sequential assembly of C5, C6, C7, C8, and multiple C9 molecules to form the MAC. C5 is the first component of the terminal pathway, and its cleavage by C5 convertase leads to the formation of C5a and C5b. C5b then interacts with C6, C7, C8, and multiple C9 molecules to form the MAC. The presence of C5 is essential for initiating this process.

**Why Each Wrong Option is Incorrect**
**Option A:** C4 is a component of the classical and alternative complement pathways but not the terminal pathway.
**Option B:** C3 is a central component of the complement system, but its cleavage leads to the formation of C3a and C3b, which then initiate the terminal pathway, not C5 itself.
**Option D:** Protein B (factor B) is a component of the alternative complement pathway, which converges with the classical pathway at the C3 level but is not directly involved in the terminal pathway.

**Clinical Pearl / High-Yield Fact**
The complement system plays a crucial role in the innate immune response, and dysregulation of this system has been implicated in various autoimmune and infectious diseases.

**✓ Correct Answer: C. C5**