An EEG obtained within 24 hours following a closed head injury would be LEAST likely to show which of the following?

An EEG obtained within 24 hours following a closed head injury is least likely to show focal spike activity. Focal spike activity typically indicates a localized irritative focus, which may emerge later in the post-trauma period as changes develop in the brain's electrical activity due to the injury.

In the immediate aftermath of a closed head injury, the brain often exhibits more diffuse changes due to the presence of edema, altered cerebral perfusion, and other acute physiological responses, which are less associated with the localized irritative phenomena indicated by focal spike activity. In contrast, asymmetry of the background activity, focal polymorphic delta activity, and slowing over the posterior head regions are more common and are often observed during this acute phase due to non-specific changes in brain function and integrity following the trauma. Therefore, while the EEG may show varying features, the presence of focal spike activity is less likely to occur soon after the injury compared to the other listed abnormalities.