**Core Concept**
The ECG lead I is a bipolar limb lead that records the electrical activity of the heart from the right arm (RA) to the left arm (LA). This lead is used to assess the QRS complex and the overall electrical axis of the heart.

**Why the Correct Answer is Right**
In a standard 12-lead ECG, lead I is a bipolar limb lead that records the difference in electrical potential between the right arm (negative electrode) and the left arm (positive electrode). This setup allows for the detection of the QRS complex, which represents the depolarization of the ventricles. The use of a bipolar lead also helps to reduce the effect of the electrical activity of the chest wall and other extracardiac sources.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because the left arm is not typically used as the negative electrode in lead I.
**Option B:** Incorrect because the left leg is typically used as the negative electrode in lead II, not lead I.
**Option C:** Incorrect because the right arm is the negative electrode in lead I, not the positive electrode.

**Clinical Pearl / High-Yield Fact**
To remember the setup of lead I, recall that the negative electrode is always on the right side (RA) and the positive electrode is always on the left side (LA) in bipolar limb leads.

**Correct Answer: C. The left arm.**