## **Core Concept**
The atrioventricular (A-V) node and the bundle of His are critical components in the electrical conduction system of the heart. The A-V node acts as a delay station for the electrical impulse, allowing the atria to fully contract before the ventricles contract. This delay is essential for coordinated pumping of the heart.

## **Why the Correct Answer is Right**
The normal total delay of the cardiac impulse in the A-V node plus the bundle of His is approximately 120 milliseconds. This delay allows sufficient time for atrial contraction and proper ventricular filling before ventricular contraction occurs. The A-V node accounts for the majority of this delay, with a normal delay of about 100 milliseconds, while the bundle of His contributes a relatively smaller portion.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option suggests a delay that is too short for the A-V node and bundle combined, as it doesn't account for the necessary delay to allow proper atrial and ventricular coordination.
- **Option B:** Similarly, this option also suggests a delay that is shorter than what is physiologically required for effective heart function.
- **Option D:** This option suggests a delay that is significantly longer than what is typically observed for the A-V node and bundle of His combined.

## **Clinical Pearl / High-Yield Fact**
A key clinical point to remember is that prolongation of the A-V nodal delay can lead to various arrhythmias and conduction disorders, such as first-degree A-V block, where the P-R interval on an ECG is prolonged (>200 milliseconds). This can be a benign finding in some individuals but may also indicate underlying heart disease.

## **Correct Answer:** . 120 ms