Which one of the following has the smallest diameter?

Options

A. trachea
B. secondary bronchiole
C. respiratory bronchiole
D. left primary bronchus

Answer

To determine which structure has the smallest diameter among the options provided, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Structures**:
- Identify the structures mentioned: larynx, trachea, primary bronchi, secondary bronchi, and respiratory bronchioles.
- Each of these structures is part of the respiratory system.

2. **Know the Function and Size of Each Structure**:
- **Larynx**: This is the voice box, but its diameter is not typically measured in the same way as the airways.
- **Trachea**: The trachea (or windpipe) is about 9 to 15 cm long and has a diameter of approximately 2 cm.
- **Primary Bronchi**: These bifurcate from the trachea and have a diameter slightly less than that of the trachea.
- **Secondary Bronchi**: These are smaller than the primary bronchi.
- **Respiratory Bronchioles**: These are the smallest airways in the lungs and continue to decrease in diameter as they branch out.

3. **Identify the Smallest Diameter**:
- As we move down the respiratory tract from the trachea to the bronchi and then to the bronchioles, the diameter decreases.
- The respiratory bronchioles are the last in this branching and therefore have the smallest diameter compared to the other structures mentioned.

4. **Conclusion**:
- Among the options provided, the **respiratory bronchioles** have the smallest diameter.

### Final Answer:
The structure with the smallest diameter is the **respiratory bronchioles**.