Which of the following changes is most useful to monitor in the child with head injury?

Monitoring the level of consciousness in a child with a head injury is crucial because it provides a direct indication of brain function and potential worsening of the injury. The level of consciousness can change rapidly following a head injury, and it is essential for assessing the severity of the injury and making decisions about treatment.

Changes in consciousness, measured through tools like the Glasgow Coma Scale, can reveal critical information about the child's neurological status. A decrease in consciousness can signify increased intracranial pressure, bleeding, or brain damage, necessitating immediate medical intervention. Monitoring this aspect allows healthcare providers to detect deterioration early and respond appropriately, which can be vital in preventing further complications.

While other factors, such as the frequency of vomiting, reflexes, and sensory exam, are also important in assessing the condition of a child with a head injury, they do not provide as immediate or direct insight into the central nervous system's status as the level of consciousness does. Therefore, focusing on consciousness monitoring is the most effective approach in this scenario.