**Core Concept**
Mobitz type I block, also known as Wenckebach block, is a type of second-degree atrioventricular (AV) block characterized by a progressive prolongation of the PR interval with each beat, culminating in a dropped beat or block. This phenomenon occurs due to a gradual delay in AV nodal conduction.

**Why the Correct Answer is Right**
Mobitz type I block is characterized by a progressive prolongation of the PR interval, with a gradual increase in the PR interval preceding the dropped beat. This is often seen on an electrocardiogram (ECG) as a sequence of PR intervals that become progressively longer, followed by a dropped beat. The mechanism behind Mobitz type I block is thought to involve a decremental conduction in the AV node, where the conduction velocity decreases with each subsequent beat.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because Mobitz type I block is indeed characterized by a progressive prolongation of the PR interval, not a sudden block.
* **Option B:** This option is incorrect because Mobitz type I block is a type of second-degree AV block, not a type of third-degree AV block.
* **Option C:** This option is incorrect because Mobitz type I block is characterized by a gradual delay in AV nodal conduction, not a sudden block in the His-Purkinje system.

**Clinical Pearl / High-Yield Fact**
Mobitz type I block is often seen in association with conditions that affect the AV node, such as myocardial infarction, digitalis toxicity, or congenital heart disease.

**Correct Answer: B. Mobitz type I block is not a type of third-degree AV block.**