Final answer:
A first-degree heart block is commonly caused by atrioventricular nodal dysfunction. It manifests as a delay in conduction on the ECG, characterized by a prolonged PR interval.
Step-by-step explanation:
The potential cause of a first-degree heart block is atrioventricular nodal dysfunction. In a first-degree heart block, there is a delay in the conduction of electrical impulses from the sinoatrial (SA) node to the atrioventricular (AV) node, which can be observed on an electrocardiogram (ECG) as an abnormally long PR interval. This delay indicates an incomplete block since the impulses still make it through to the ventricles, just at a slower rate. Conditions such as enhanced sympathetic activity would typically increase the rate of conduction rather than delay it. A bundle branch block implies an obstruction further down the conduction pathway in the left or right bundle branches and would not alone cause a first-degree AV block.