Which rhythm features cyclic lengthening of the PR interval?

The rhythm characterized by cyclic lengthening of the PR interval is known as 2nd Degree Type 1 Block, also referred to as Mobitz Type I. In this type of block, the PR interval progressively lengthens with each heartbeat until a beat is dropped (i.e., a QRS complex is absent). This gradual elongation occurs in a cyclical pattern, which allows for the identification of the phenomenon on an electrocardiogram (ECG).

In contrast, 1st Degree AV Block features a consistently prolonged PR interval without the cyclical lengthening seen in Type 1 block. Also, 2nd Degree Type 2 Block does not exhibit the same progression and is marked by drops of beats without a preceding lengthening of the PR interval. Meanwhile, Atrial Fibrillation is characterized by an irregularly irregular rhythm and does not involve a periodic change in PR interval at all. Therefore, the distinct characteristic of progressive lengthening leading to a dropped beat in 2nd Degree Type 1 Block makes it the correct answer.