Which lead represents the left arm and detects positive deflection due to electrical activity?

The correct answer, which represents the left arm and detects positive deflection due to electrical activity, is associated with aVL. Lead aVL, or augmented vector left, is specifically designed to measure the electrical activity from the left arm relative to a reference point between the other limbs. This lead provides critical information about the electrical forces directed towards the left arm, making it particularly useful in assessing the lateral wall of the heart.

In aVL, when the heart's electrical activity is directed towards the left arm, it results in a positive deflection on the ECG reading. This is because aVL creates a positive vector from the left arm while averaging the electrical potentials from the right arm and the feet. Thus, any depolarization occurring in the heart that moves towards the left arm results in a positive response in this lead.

Other leads mentioned may measure different aspects of cardiac activity but do not focus specifically on the left arm's predominant electrical influences in the same way as aVL does. For instance, options that measure the right side or inferior aspects of the heart, would not be able to reflect the electrical activity directed towards the left arm, which is crucial for accurate interpretation of ECG readings.