Final answer:
The nurse should prioritize observing for excessive dryness of the mucus membranes when providing oral care to Mrs. McElroy, as this can indicate dehydration or effects of the antiemetic.
Step-by-step explanation:
When providing oral care to Mrs. McElroy, the nurse's priority assessment should be to observe for excessive dryness of the mucus membranes. This observation is essential as it could indicate dehydration or a side effect of the antiemetic medication. It is also a necessary part of providing mouth care, especially after a patient experiences nausea and possible vomiting.
In this scenario, while the other assessments could also be important, they are not as directly related to the administration of an antiemetic and the immediate care being provided. For instance, enlarged cervical lymph nodes could indicate an infection or other health issues, but are not the priority during oral care. Palpating the thyroid gland can be necessary in a broader physical examination, and while checking for the presence of cavities is part of dental health, it is not pertinent to the nausea treatment.