## **Core Concept**
The oesophagus has three main constrictions that are clinically significant, particularly during the passage of instruments like an endoscope. These constrictions are located at specific anatomical landmarks.

## **Why the Correct Answer is Right**
The third constriction of the oesophagus is located at the **diaphragmatic hiatus**, which is where the oesophagus passes through the diaphragm. This is a crucial point because it represents a physiological narrowing where the oesophagus traverses the diaphragm at the oesophageal hiatus, typically at the level of T10.

## **Why Each Wrong Option is Incorrect**
- **Option A:** The first constriction is at the **cricopharyngeal sphincter** (or upper oesophageal sphincter), which is not the third constriction.
- **Option B:** The second constriction is where the **left main bronchus** and the **arch of the aorta** cross the oesophagus, not the third constriction.
- **Option C:** This option might seem plausible but is not specified; however, based on the elimination and knowledge of the first two constrictions, we focus on the correct third constriction.

## **Clinical Pearl / High-Yield Fact**
A key clinical point to remember is that the three constrictions of the oesophagus are at the:
1. Cricopharyngeal sphincter
2. Crossing of the aortic arch and left main bronchus
3. Diaphragmatic hiatus

These are critical points for the passage of an endoscope and for understanding potential sites of oesophageal obstruction.

## **Correct Answer:** D. Diaphragmatic hiatus.