Choice A rationale:

Assessing blood glucose levels is not directly related to ondansetron administration for vomiting. Ondansetron does not typically affect blood glucose levels.

Choice B rationale:

Obtaining a stool sample for culture and microscopy is important for diagnosing the cause of gastroenteritis, but it is not a necessary step before administering ondansetron.

Choice C rationale:

Monitoring urinary output is not directly associated with the administration of ondansetron. It's important for assessing hydration status, but it's not the immediate concern when administering this antiemetic medication.

Choice D rationale:

Checking vital signs is essential before giving any medication to ensure the client's overall stability and to detect any potential contraindications or adverse reactions.