## Core Concept
The measurement of intravascular pressure by a pulmonary artery catheter is a critical procedure in critical care medicine. It involves the insertion of a catheter into the pulmonary artery to assess pressures within the heart and lungs. This procedure is typically performed to guide management in critically ill patients.

## Why the Correct Answer is Right
The correct phase for measuring intravascular pressure by a pulmonary artery catheter is during **end-expiration**. This is because during end-expiration, the intrathoracic pressure is closest to atmospheric pressure, providing a more accurate measurement of the pressures within the heart and pulmonary artery without the confounding effects of positive intrathoracic pressure changes that occur during inspiration and expiration. Measurements taken during this phase offer a clearer reflection of the cardiac pressures.

## Why Each Wrong Option is Incorrect
- **Option A:** Early inspiration - Incorrect because early inspiration can cause a decrease in intrathoracic pressure, potentially altering the measured pressures.
- **Option B:** Peak inspiration - Incorrect because peak inspiration increases intrathoracic pressure significantly, which can inaccurately elevate measured pressures.
- **Option C:** Early expiration - While closer to the correct answer, early expiration might not provide as stable a measurement as end-expiration due to minor fluctuations in intrathoracic pressure.

## Clinical Pearl / High-Yield Fact
A key point to remember is that accurate measurement of pressures via a pulmonary artery catheter is best achieved during **end-expiration** to minimize the impact of respiratory cycle-related changes in intrathoracic pressure. This ensures that the measured pressures are reflective of the true cardiac function rather than being influenced by the respiratory cycle.

## Correct Answer: D. End-expiration