Final answer:
The nurse should prioritize asking if the woman is sexually active and if she could be pregnant, as this would help diagnose or rule out pregnancy-related conditions. This question is crucial due to the urgent need to address potential emergencies such as ectopic pregnancy.
Step-by-step explanation:
The most useful question from the nurse to determine the cause of a 26-year-old woman's vomiting and abdominal pain would be "Are you sexually active, and could you be pregnant?" The symptoms described can have various causes, but considering the age and potential for pregnancy, this question can help to rule in or out pregnancy-related conditions such as early pregnancy symptoms or complications like an ectopic pregnancy. Additionally, the timing of symptoms in relation to the menstrual cycle and sexual activity can provide valuable diagnostic clues.
Other potential causes for such symptoms that may be considered depending on the patient's history and further details include food poisoning, stress-related gastrointestinal issues, infections, and allergic reactions. However, given that ectopic pregnancy, for example, is a medical emergency, confirming or excluding pregnancy-related issues would likely be prioritized in this clinical scenario.