What is the most likely cause of left axis deviation observed in an ECG?

Left axis deviation on an ECG typically indicates a shift in the electrical activity of the heart, most often due to structural changes in the left ventricle or left atrium. In this context, hypertrophy of the left ventricle is a common cause, as it increases the muscle mass and alters the conduction pathways related to the heart’s electrical activity. This hypertrophy can arise from various conditions, such as hypertension or valvular heart disease, leading to the leftward shift in the heart's electrical axis.

The presence of left axis deviation suggests that the heart's electrical activity is oriented more towards the left side, which aligns with the characteristics of left ventricular hypertrophy. This is a direct correlation, making hypertrophy the most logical choice when considering the most likely cause of left axis deviation on an ECG.