What causes a stiff, narrowed aortic valve?

The most common cause of a stiff, narrowed aortic valve (aortic stenosis) is a buildup of scar tissue and calcium on the valve. Factors that contribute to narrowing of this valve include age, an inborn valve defect, and various diseases such as kidney disease. Aortic stenosis can cause shortness of breath, fatigue, and lightheadedness, especially during physical activity.