## Core Concept
Second-degree AV block is characterized by the intermittent failure of the atrioventricular (AV) node to conduct atrial impulses to the ventricles. This condition can be classified into two types: Mobitz type I (Wenckebach) and Mobitz type II. The key features include dropped beats, where a P wave is not followed by a QRS complex.

## Why the Correct Answer is Right
In second-degree AV block, particularly in Mobitz type I, there's a progressive prolongation of the PR interval until a beat is dropped. This indicates an **increase in AV conduction time** before the block occurs. The **change in morphology of the ventricular complex** isn't a defining feature but the dropped beats and variable PR intervals are. The **increased atrial rate compared to ventricular rate** is a hallmark because some atrial impulses are not conducted to the ventricles.

## Why Each Wrong Option is Incorrect
* **Option C: Increase in cardiac output** - This is incorrect because second-degree AV block can lead to a decrease in cardiac output due to the decreased ventricular rate.
* **Option D: Decrease in stroke volume** - While this might occur due to the decreased rate, it's not a direct or specific characteristic used to define second-degree AV block.
* **Option E: Increased AV conduction time** - This is partially correct in the context of Wenckebach (Mobitz I), but the question seems to seek more direct associations with second-degree AV block.

## Clinical Pearl / High-Yield Fact
A key clinical pearl is that in **Mobitz type II second-degree AV block**, the PR interval is constant before the dropped beat, distinguishing it from Mobitz type I. This condition often progresses to complete heart block.

## Correct Answer: B.