**Core Concept**
The cardiac conduction system is responsible for regulating the heart rate and coordinating the contraction of cardiac muscle cells. It consists of specialized fibers that transmit electrical impulses from the sinoatrial (SA) node to the ventricles, enabling coordinated heartbeats. The conduction velocity varies among different parts of the cardiac conduction system.

**Why the Correct Answer is Right**
The AV node is the slowest part of the cardiac conduction system, with a conduction velocity of about 0.05 m/s. This is because the AV node is responsible for delaying the electrical impulse to allow for proper filling of the ventricles before contraction. The delay is essential for ensuring efficient cardiac function. The AV node contains specialized fibers that are rich in gap junctions, which are essential for electrical conduction. However, the AV node also contains a high concentration of potassium channels, which contribute to its slow conduction velocity.

**Why Each Wrong Option is Incorrect**
**Option A:** The SA node is the fastest part of the cardiac conduction system, with a conduction velocity of about 1 m/s. It is responsible for initiating the heartbeat and does not have a significant delay in conduction.
**Option B:** The Bundle of His is a bundle of specialized fibers that transmits the electrical impulse from the AV node to the ventricles, with a conduction velocity of about 2 m/s. It is faster than the AV node but slower than the SA node.
**Option C:** The Purkinje fibers are specialized fibers that transmit the electrical impulse from the Bundle of His to the ventricular muscle cells, with a conduction velocity of about 4 m/s. They are the fastest part of the cardiac conduction system.

**Clinical Pearl / High-Yield Fact**
It is essential to recognize that the AV node is the rate-limiting step in cardiac conduction, and its dysfunction can lead to arrhythmias such as AV block. Understanding the conduction velocity of different parts of the cardiac conduction system is crucial for diagnosing and managing cardiac rhythm disorders.

**Correct Answer:** D. The AV node is the slowest part of the cardiac conduction system.