Lead III records electrical activity between which two electrodes?

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

Answer: (B)

Lead III measures the heart’s electrical activity along the axis from the left arm to the left leg. In this lead, the left leg electrode is positive and the left arm electrode is negative, so the signal reflects the voltage difference between the left arm and the left leg. That makes the two electrodes involved the left arm and the left leg.

This limb-lead setup is one part of Einthoven’s triangle, where the three standard limb leads pair different arms and leg electrodes to capture different views of the heart’s electrical activity. The other common pairs correspond to the right arm with the left leg (Lead II) and the right arm with the left arm (Lead I), which is why those options describe different leads.