How is a 2nd degree AV block (Mobitz II) identified?

In a 2nd degree AV block, specifically Mobitz II, the distinguishing feature is that the P waves are conducted normally until a certain point, at which a QRS complex is dropped without prior warning. This means that the electrical signals (P waves) are entering the AV node and some reach the ventricles, but occasionally, one of the signals does not generate a QRS complex.

The defining aspect of Mobitz II is that the PR intervals remain constant and are not gradually increasing before a drop occurs, which differentiates it from Mobitz I (Wenckebach). In Mobitz II, there are consistent PR intervals, and the dropped QRS occurs unpredictably. Therefore, the answer correctly identifies a situation where P waves are generated but fail to be conducted through to the ventricles, resulting in a dropped QRS without a preceding irregularity in the PR interval.

The other options describe characteristics of AV blocks incorrectly or misinterpret the nature of Mobitz II block. For instance, a regular PR interval with a dropped QRS does not specifically convey the unique attributes of Mobitz II, while the other choices refer to situations or types of blocks that do not pertain to this specific type of 2nd degree AV block.