Lead III records electrical activity between which two electrodes?

• A. Right arm to left leg
• B. RA to LA
• C. Left arm to left leg
• D. Left leg to RA

Answer: (C)

Lead III is one of the standard limb leads that compares voltages from pairs of limb electrodes. In this lead, the left leg serves as the positive electrode and the left arm serves as the negative electrode, so it records electrical activity between the left leg and the left arm. In other words, it measures the potential difference along the axis from the left arm toward the left leg (the vector LL minus LA). This distinguishes it from the other limb leads, which pair different limbs.