**Core Concept**
Second-degree heart block (Mobitz type I or II) involves partial failure of atrioventricular (AV) conduction, where not all atrial impulses are conducted to the ventricles. This results in a variable PR interval and dropped or delayed ventricular beats, reflecting impaired conduction through the AV node.

**Why the Correct Answer is Right**
In second-degree heart block, **prolonged conduction time** is evident as the PR interval increases progressively (in Mobitz I), or remains constant with dropped beats (in Mobitz II). **Atrial rate is typically faster than ventricular rate** because atrial impulses are generated at a normal rate but not all are conducted, leading to a slower ventricular response. **Change in QRS morphology** may occur, especially in Mobitz II, where bundle branch blocks or ventricular escape rhythms are present, altering the QRS complex. Thus, all listed features are clinically observable.

**Why Each Wrong Option is Incorrect**
Option A: While QRS morphology may change, it is not a universal feature—only in advanced cases or associated bundle branch blocks. However, since it *can* occur in second-degree block, it is not incorrect.
Option B: This is correct—atrial rate exceeds ventricular rate due to AV dissociation.
Option C: Prolonged conduction time is central to second-degree block, especially in Mobitz I.
Option D: All features are valid, so D is correct.

Note: Since the question asks to select *three* options and D is "all of the above," selecting D implies all are correct. Hence, D is the only valid choice.

**Clinical Pearl / High-Yield Fact**
In Mobitz II, the PR interval remains constant before a dropped beat, and QRS morphology may be abnormal—this is a key differentiator from Mobitz I. A slow ventricular rate and AV dissociation are hallmark signs.

✓ Correct Answer: D. All of the above