Constant PR inverval is seen in -a) First degree blockb) Second degree – Mobiz type I block.c) Second degree – Mobiz type II blockd) Third degree block

**Core Concept**
Constant PR interval is a characteristic feature of a specific type of arrhythmia that affects the conduction of electrical impulses through the atrioventricular (AV) node. This phenomenon is related to the properties of the AV node, which can lead to a consistent delay in the transmission of impulses from the atria to the ventricles.

**Why the Correct Answer is Right**
The correct answer is related to the Mobitz type I second-degree AV block, also known as Wenckebach block. In this condition, there is a progressive prolongation of the PR interval with each subsequent beat, until a beat is dropped. This occurs due to a gradual increase in the refractory period of the AV node, which is caused by a progressive delay in the conduction of impulses through the AV node. The PR interval becomes constant because the dropped beat is not counted in the cycle length, resulting in a consistent interval between the P wave and the next QRS complex.

**Why Each Wrong Option is Incorrect**
**Option A:** First-degree block is characterized by a persistent prolongation of the PR interval, but it does not involve the dropping of beats, which is a hallmark of second-degree block.

**Option B:** Mobitz type I block (Wenckebach block) is the correct answer, so this option is incorrect by definition.

**Option D:** Third-degree block, also known as complete heart block, involves a complete failure of conduction between the atria and ventricles, resulting in a complete dissociation between the P wave and the QRS complex. This does not involve a constant PR interval.

**Clinical Pearl / High-Yield Fact**
The Wenckebach block is named after the German physician Karl Wenckebach, who first described this condition in 1895. It is a classic example of a second-degree AV block, and its diagnosis can be made by observing the progressive prolongation of the PR interval on an electrocardiogram (ECG).

**Correct Answer: C. Second degree - Mobiz type II block.**