Final answer:
The priority adverse effect reported by the guardian that the nurse should report to the provider is the elevated blood pressure, as it poses a significant health risk compared to other side effects.
Step-by-step explanation:
When assessing the adverse effects of atomoxetine reported by the guardian of a school-age child, it is crucial to identify and prioritize the symptoms that may pose the most significant health risk. Of the listed side effects, elevated blood pressure is the most concerning and should be reported to the provider immediately. Atomoxetine is a medication commonly used to treat Attention-Deficit/Hyperactivity Disorder (ADHD), and it may cause various side effects such as agitation, anxiety, or sleep problems; however, an elevation in blood pressure can be particularly dangerous as it may lead to serious cardiovascular events. Therefore, elevated blood pressure stands out as a priority adverse effect among others like decreased appetite, insomnia, and irritability, which are generally less severe.