Which of the following best describes the nature of acute side effects?

Acute side effects are characterized by their timing in relation to the treatment regimen. They typically occur during or shortly after the treatment process, often arising from the rapid changes that the treatment induces in the body's tissues or systems. This is in contrast to side effects that may develop long after treatment has concluded, which are defined as late or chronic effects, not acute.

The nature of these side effects can often be temporary, resolving within a short period after the treatment has ended. They are not typically permanent or progressive forms of damage, and while some acute side effects can be serious, they are not always life-threatening. Consequently, the best descriptor among the available choices is that acute side effects occur during treatment.