Final answer:
The statement that a consistently lengthened PR interval indicates a first-degree AV block is true, reflecting delayed conduction between the SA and AV nodes on an ECG. The correct option is a).
Step-by-step explanation:
First-degree AV block is characterized by a consistently lengthened PR interval on an electrocardiogram (ECG), which is a sign of delayed conduction between the sinoatrial (SA) node and the atrioventricular (AV) node. The statement that a consistently lengthened PR interval indicates a first-degree AV block is true.
In contrast, a second-degree AV block would show some P waves not followed by a QRS complex, indicating that some impulses fail to conduct through the AV node, while in a third-degree AV block, there is no correlation between the P waves and QRS complexes, signaling a complete block of conduction.
It is critical to recognize these distinctions as they have varying clinical implications and management strategies. Option a) is the correct one.