**Core Concept**
The bronchial tree is a complex network of airways that branch off from the trachea, leading to the lungs. Bronchoscopy is a procedure used to visualize the airways, allowing for the examination of the trachea, bronchi, and bronchioles.

**Why the Correct Answer is Right**
A bronchoscope is a flexible or rigid tube with a camera and light on the end, used to visualize the airways. During a bronchoscopy, the bronchoscope can be inserted through the mouth or nose, and the airways can be examined for abnormalities. However, there are certain structures that are not visible during a bronchoscopy due to their location or size.

**Why Each Wrong Option is Incorrect**
**Option A:** The trachea is visible during bronchoscopy, as it is the main airway that leads to the lungs.
**Option B:** The bronchi and bronchioles are also visible during bronchoscopy, as they branch off from the trachea and lead to the lungs.
**Option C:** The carina is a ridge of cartilage that separates the right and left main bronchi, and it is visible during bronchoscopy.

**Clinical Pearl / High-Yield Fact**
The carina is an important landmark during bronchoscopy, and it can be used to guide the bronchoscope into the correct airway. The carina is also a common site for tumors and other abnormalities.

**Correct Answer: D. Pulmonary Artery**