Which of the following steps is NOT part of the intrinsic pathway of apoptosis?

The intrinsic pathway of apoptosis is primarily regulated by mitochondrial signals and includes various key processes such as the release of cytochrome c from the mitochondria into the cytosol, which activates downstream apoptotic pathways. This release is essential for the activation of caspases, which are the executioners of apoptosis that lead to the cell's death.

p53, a crucial tumor suppressor protein, is involved in the intrinsic pathway by promoting apoptosis in response to DNA damage. It can trigger the expression of pro-apoptotic factors and regulate the anti-apoptotic Bcl-2 family proteins, further facilitating mitochondrial outer membrane permeabilization and the release of cytochrome c.

In contrast, fibrinogen synthesis is unrelated to the intrinsic pathway of apoptosis. Fibrinogen is a plasma protein involved in blood coagulation rather than in the apoptotic process. It is synthesized in the liver and plays a role in forming the fibrin clot during hemostasis, making it unrelated to the mechanisms governing apoptosis.

Thus, understanding the components of the intrinsic pathway clarifies why the release of cytochrome c, the activation of caspases, and p53 activation are integral parts of the pathway, while fibrinogen synthesis does not belong