Final answer:
A second-degree atrioventricular heart block is characterized by some impulses from the SA node reaching the AV node and continuing, while others do not. An example of a second-degree atrioventricular heart block is not (C) Third-degree heart block where there is no correlation between atrial activity (the P wave) and ventricular activity (the QRS complex).
Step-by-step explanation:
A second-degree atrioventricular heart block is characterized by some impulses from the SA node reaching the AV node and continuing, while others do not. This can be recognized on an ECG as some P waves not followed by a QRS complex, while others appear normal.
Out of the given options, the not an example of a second-degree atrioventricular heart block is (C) Third-degree heart block. In a third-degree block, there is no correlation between atrial activity (the P wave) and ventricular activity (the QRS complex).