Final answer:
The highest priority step in the accurate use of a tympanic thermometer is ensuring proper placement in the ear canal.
Step-by-step explanation:
The highest priority step in the accurate use of a tympanic thermometer is ensuring proper placement in the ear canal. These modern thermometers use infrared technology to measure body temperature and are more accurate than other methods such as using alcohol thermometers under the tongue or in the armpit. Placing the thermometer correctly in the ear canal allows for the most accurate measurement of body temperature.