Final answer:
The first step the medical assistant should take to ensure the validity of the blood pressure reading is to take the other vital signs and recheck the blood pressure after a few minutes.
Step-by-step explanation:
The first step the medical assistant should take to ensure the validity of the blood pressure reading is to take the other vital signs and recheck the blood pressure after a few minutes. This is because blood pressure can fluctuate due to various factors such as hydration, stress, and time of day. By taking other vital signs and rechecking the blood pressure, the medical assistant can determine if the initial reading was accurate or if there were any factors that may have influenced the elevated reading.