Which condition is NOT a defect associated with heart valves?

Atrial fibrillation is the correct choice because it is not a defect associated with heart valves. Instead, atrial fibrillation is an arrhythmia, a condition characterized by an irregular and often rapid heartbeat caused by disorganized electrical signals in the heart's atria. This condition can lead to various complications, such as stroke and heart failure, but it does not represent a structural defect in the heart valves themselves.

In contrast, prolapse, stenosis, and insufficiency are all specific defects that can affect the heart valves. Prolapse refers to a condition where a valve does not close properly, leading to backflow of blood. Stenosis involves the narrowing of a valve, which restricts blood flow and increases the workload on the heart. Insufficiency, also known as regurgitation, occurs when a valve fails to close completely, allowing blood to flow backward. Each of these conditions represents a malfunction of heart valves, whereas atrial fibrillation arises from electrical disturbances rather than structural issues.