Which of the following is NOT a vital sign monitored during anesthesia?

Monitoring vital signs during anesthesia is crucial for ensuring the safety and health of the patient. Among the options provided, pupil size is typically not classified as a vital sign. Vital signs are measurements that provide essential information regarding the physiological status of an animal during anesthesia, which includes heart rate, respiratory rate, and core body temperature.

Heart rate is critical as it reflects cardiac output and the overall cardiovascular status. Respiratory rate communicates the effectiveness of gas exchange and the animal's respiratory function. Core body temperature is vital as it indicates the metabolic and physiological balance; hypothermia can be a common issue during anesthesia.

While pupil size can indicate certain neurological responses and the depth of anesthesia, it is not routinely classified as a vital sign. Therefore, it is correct that pupil size is the option that is NOT categorized as a vital sign to be monitored during anesthesia.