Which best characterizes the membrane attack complex (MAC)?

• A. C9 polymerizes to form the transmembrane channel.
• B. Each pathway uses different factors to form it.
• C. One MAC unit is sufficient to lyse any type of cell.
• D. C5 through C9 are not added in any particular order.

Answer: (A)

The key idea is how the membrane attack complex creates a pore in the target membrane. After the early steps of the complement cascade, the terminal assembly uses C5b along with C6, C7, C8, and multiple C9 units. It is the polymerization of C9 that actually constructs the transmembrane channel. This pore disrupts membrane integrity, allowing ions and water to flow freely and leading to cell lysis. The other statements don’t fit because the MAC is formed through a defined, sequential assembly that converges from all three pathways, one MAC pore is not sufficient to lys the cell, and C9 polymerization is the crucial step that makes the pore.