Final answer:
The nurse should instruct parents to report generalized urticaria immediately, as it may indicate an anaphylactic reaction, which is life-threatening and requires urgent medical intervention.
Step-by-step explanation:
Among the potential adverse effects after immunization, the one that the nurse should instruct parents to report immediately is generalized urticaria. This is because generalized urticaria, or widespread hives, is indicative of a possible severe allergic reaction, such as anaphylaxis. Anaphylaxis is a life-threatening condition that requires prompt medical attention. Contrastingly, pain and local swelling at the injection site are common minor reactions post-immunization, and a mild temperature elevation is not uncommon and is typically not a cause for immediate concern, unless it becomes high or persistent.
The nurse should emphasize the importance of monitoring for signs of severe allergic reactions, such as difficulty breathing, swelling of the face and throat, a significant drop in blood pressure, or widespread hives. In the event of these symptoms, applying an epinephrine autoinjector and seeking emergency medical care is crucial.