What is the reason that a pulp calcifies after trauma?

The correct answer highlights that a low-intensity blow may not cause immediate pulp death, but can lead to a physiological response known as pulp calcification. When trauma occurs, the dental pulp can react by producing more dentin (the calcified tissue that surrounds the pulp chamber). This reaction is often a protective mechanism to shield the pulp from potential damage or infection, as the formation of additional dentin can help in sealing off the pulp chamber.

Trauma does not always lead to irreversible damage; instead, it can result in a reparative process where the dentin reacts positively. Consequently, when the blow is not severe enough to sever the blood supply completely or expose the pulp to bacteria, the pulp may undergo a process of calcification rather than necrosis. This pulp calcification can help in maintaining the integrity and health of the tooth despite the impact of the trauma.

Options connected to irreversible damage, complete severing of the blood supply, or exposure to bacteria suggest more severe implications for the pulp, which are not synonymous with the scenario presented in the question where a low-intensity blow leads to calcification instead.