**Core Concept**
Mobitz Type I block, also known as Wenckebach block, is a type of second-degree atrioventricular (AV) block characterized by progressive prolongation of the PR interval with each subsequent beat, ultimately resulting in a dropped beat. This phenomenon occurs due to intermittent failure of conduction through the AV node, leading to varying degrees of AV block.

**Why the Correct Answer is Right**
The hallmark feature of Mobitz Type I block is the progressive prolongation of the PR interval, which is not constant. This is because the block is intermittent, and the PR interval shortens with each beat until a dropped beat occurs. The reason for this phenomenon is the gradual decremental conduction through the AV node, which involves the sequential failure of conduction through the AV nodal pathways.

**Why Each Wrong Option is Incorrect**
**Option B:** Normal QRS morphology is not a defining feature of Mobitz Type I block. However, the QRS morphology may appear normal if the block occurs at the AV junction, but it can be abnormal if the block occurs at the bundle branches or Purkinje fibers.
**Option C:** Regular atrial rhythm is a feature of Mobitz Type I block, as the block affects the AV conduction rather than the atrial rhythm.
**Option D:** Atrial rate-ventricular rate difference is a feature of Mobitz Type I block, as the block results in intermittent dropped beats, leading to a difference between the atrial and ventricular rates.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that Mobitz Type I block is often asymptomatic and may be discovered incidentally on an electrocardiogram (ECG). However, it can be a precursor to more severe forms of AV block, such as third-degree AV block, which requires prompt medical attention.

**✓ Correct Answer: A. Constant PR interval.**