Which type of heart block is characterized by a progressive lengthening of the PR interval until a QRS is dropped?

The type of heart block that features a progressive lengthening of the PR interval until a QRS complex is dropped is known as Second-degree AV block Type I, commonly referred to as Wenckebach. This block specifically involves a gradual prolongation of the PR interval with each successive beat, culminating in a beat where the QRS complex does not occur following the final long PR interval.

This phenomenon occurs due to a failure in the conduction of impulses through the AV node, often related to increased vagal tone or other factors that affect the node's conductivity. The hallmark of this block is the pattern of progressively lengthening PR intervals, which distinguishes it from other types of heart blocks.

In contrast, first-degree AV block is characterized by a constant PR interval, while second-degree AV block Type II is identified by dropped beats without a pattern of progressive PR interval lengthening. Third-degree block, or complete heart block, results in complete dissociation between atrial and ventricular activity, without any predictable relationship between them. Therefore, the specific characteristic of progressive PR interval lengthening culminating in a dropped QRS complex is what defines Wenckebach (Second-degree AV block Type I) as the correct answer to the question.