What is the potential risk of untreated atrial fibrillation?

The potential risk of untreated atrial fibrillation is closely associated with stroke due to thrombus formation. Atrial fibrillation is characterized by disorganized electrical activity in the atria, which results in ineffective atrial contractions. This ineffectiveness can lead to blood stagnation within the heart, particularly in the left atrial appendage. When blood flow is sluggish, it can promote the development of thrombi (blood clots). If a thrombus forms and subsequently embolizes, it can travel to the brain, resulting in an ischemic stroke.

The recognition of this risk is critical in managing individuals with atrial fibrillation. Anticoagulation therapy is often prescribed to mitigate the risk of thrombus formation and subsequent stroke, especially in patients with additional stroke risk factors.

In terms of the other options, while heart failure, increased heart rate, and decreased exercise tolerance can all be consequences of atrial fibrillation, they do not pose the immediate and severe risk that stroke presents. Therefore, the most significant and dangerous complication of untreated atrial fibrillation is indeed the risk of stroke due to thrombus formation.