Final answer:
The triage nurse should see the infant with vomiting and diarrhea first, as infants are more susceptible to dehydration and rapid changes in vital signs, which can be life-threatening.
Step-by-step explanation:
The triage nurse's priority in the emergency department is to see the client whose condition is the most urgent and who cannot wait without risk of serious harm. In the provided scenarios, the infant with vomiting and diarrhea should be seen first. Infants are more susceptible to dehydration and changes in blood pressure and heart rate from gastrointestinal symptoms. These conditions can quickly become dangerous if not treated promptly. In contrast, nausea and vomiting in a young adult for several hours, vaginal spotting at 8 weeks gestation, and a low-grade fever in a toddler are generally less immediately life-threatening and can be seen after the infant.