Final answer:
The correct answer is option C. The nurse's crucial first assessment should be to determine if the child's throat itches, as this can indicate an allergic reaction and potential anaphylaxis, which is more immediately life-threatening than other symptoms.
Step-by-step explanation:
The school nurse's most important assessment for a child who feels strange after switching lunches with a friend would be C) Determining if her throat itches.
This is crucial because an itchy throat could be an early sign of an allergic reaction, especially if the child ingested something they are allergic to.
Immediate attention to symptoms like throat itchiness can be lifesaving as it might lead to anaphylaxis, a severe allergic reaction that requires urgent medical attention.
Other symptoms such as a rash, nausea, and abdominal pain are also important but are less indicative of a potential airway-compromising emergency.