Which of the following is NOT a standard lead used in a 12-lead ECG?

In a 12-lead ECG, standard leads consist of a specific configuration that includes limb leads and precordial leads. Leads I, II, and III are considered standard limb leads and are derived from the placement of electrodes on the limbs, specifically measuring the electrical activity of the heart from different angles.

Lead I measures the potential difference between the right arm and the left arm, Lead II measures the difference between the right arm and the left leg, and Lead III measures the difference between the left arm and the left leg. These leads provide important information about the heart's electrical activity.

On the other hand, Lead V is not a standard lead in the sense of being a limb lead. Instead, it represents a precordial lead, which is derived from electrodes placed on the chest. The standard precordial leads generally include V1 to V6, which are positioned at specific anatomical locations on the chest to provide detailed views of the heart’s electrical activity from the horizontal plane.

Therefore, the designation of Lead V as a non-standard lead in the context of the question underscores the distinction between limb leads, which are classified as Leads I, II, and III, and precordial leads, which include V1 through V6. This