Final answer:
To measure the blood flow velocity in the femoral artery specifically, the ultrasound operator should adjust the Pulse Repetition Frequency (PRF) to correctly position the gate and avoid picking up signals from the femoral vein.
Step-by-step explanation:
The ultrasound operator who is attempting to measure the velocity of flow within the femoral artery but is incidentally picking up signals within the femoral vein would need to adjust the position of the gate by changing the Pulse Repetition Frequency (PRF). The PRF control allows the sonographer to discriminate between blood flow in vessels at different depths, which helps in selectively measuring the velocity in the desired vessel, such as the femoral artery, without interference from adjacent vessels like the femoral vein.