**Core Concept**
Left axis deviation (LAD) on an electrocardiogram (ECG) indicates a problem with the electrical conduction system of the heart, specifically involving the left side. This can be due to various conditions affecting the left ventricle or its blood supply, such as a myocardial infarction (MI) or left ventricular hypertrophy.

**Why the Correct Answer is Right**
Left axis deviation is characterized by a QRS axis between -30° and -90°, which means the electrical impulse is being conducted abnormally to the left side of the heart. This can lead to inefficient heart function and potential complications such as heart failure or arrhythmias. The QRS axis is measured by determining the angle at which the electrical impulses pass through the ventricles, and in LAD, this angle is shifted to the left.

**Why Each Wrong Option is Incorrect**
**Option A:** Normal axis deviation is typically between +90° and -30°, so an axis between -30° and -90° indicates a problem.
**Option B:** Right axis deviation (RAD) is characterized by a QRS axis between +90° and +180°, which is the opposite of LAD.
**Option C:** A QRS axis of 0° would indicate a vertical axis, which is not a normal or abnormal axis deviation.

**Clinical Pearl / High-Yield Fact**
When interpreting an ECG, it's essential to note that left axis deviation can be a sign of a more severe underlying condition, such as a large MI or significant left ventricular hypertrophy, and requires further evaluation.

**Correct Answer: C. A QRS axis of 0° would indicate a vertical axis, which is not a normal or abnormal axis deviation.**