Heart block is the failure of stimulation to the ventricles following atrial contraction. Which one of the following heart structures could not be involved in heart block?

Options

A. Atrioventricular node
B. Bundle of his fibers
C. Purkinje fibers
D. sinoatrial node

Answer

To determine which heart structure could not be involved in heart block, we need to analyze the role of each structure mentioned in the question.

### Step-by-Step Solution:

1. **Understanding Heart Block**:
- Heart block refers to the failure of electrical impulses to properly stimulate the ventricles after atrial contraction. This results in irregular heartbeats.

2. **Identifying the Structures**:
- The question lists four structures:
- Atrioventricular (AV) node
- Bundle of His fibers
- Purkinje fibers
- Sinoatrial (SA) node

3. **Role of Each Structure**:
- **Sinoatrial (SA) Node**:
- Known as the natural pacemaker of the heart, it generates electrical impulses that initiate the heartbeat. It is involved in heart block as it is responsible for starting the conduction process.

- **Atrioventricular (AV) Node**:
- This node connects the atria to the ventricles and is crucial for transmitting impulses from the atria to the ventricles. If there is a block, it would directly affect the conduction to the ventricles.

- **Bundle of His**:
- This structure transmits impulses from the AV node to the Purkinje fibers. It plays a significant role in the conduction pathway and can be involved in heart block.

- **Purkinje Fibers**:
- These fibers are responsible for the contraction of the ventricles. They receive impulses from the Bundle of His and can also be affected by heart block.

4. **Conclusion**:
- Since the SA node initiates the heartbeat and the AV node, Bundle of His, and Purkinje fibers are all involved in the conduction pathway to the ventricles, the structure that could not be involved in heart block is the **Sinoatrial (SA) Node**.

### Final Answer:
The structure that could not be involved in heart block is the **Sinoatrial (SA) Node**.