Which arrhythmia is characterized by "dropped beats" without a progressive lengthening of the PR interval?

The arrhythmia that features "dropped beats" without a progressive lengthening of the PR interval is indeed second-degree AV block, Type II. In this condition, the conduction through the AV node is intermittent, leading to occasional failure of the conduction to the ventricles, resulting in dropped beats. Unlike Type I (Wenckebach) where there is a gradual lengthening of the PR interval before a beat is dropped, in Type II, the PR interval remains constant for conducted beats. The presence of consistent PR intervals followed by sudden drops of beats distinguishes this type of block from others.

In this scenario, first-degree AV block is characterized by a prolonged PR interval but no dropped beats, while third-degree AV block (complete heart block) involves a complete dissociation between atrial and ventricular activity, with more severe implications than just dropped beats. Therefore, the hallmark of second-degree AV block, Type II is identifiable by the condition of having consistent PR intervals with sporadic dropped QRS complexes.