## Core Concept
The question assesses the understanding of plasma volume evaluation methods. Plasma volume can be measured using various techniques, including dilution methods with specific markers. The ideal marker should remain confined to the vascular space and not leak into the interstitial space.

## Why the Correct Answer is Right
The correct answer, **D. Albumin**, is right because albumin is a large protein that remains primarily within the vascular space, making it an excellent marker for measuring plasma volume. When albumin labeled with a radioactive isotope (e.g., ¹²⁵I) is injected into the bloodstream, it distributes throughout the plasma compartment but does not significantly leak into the interstitial space. This property allows for accurate measurement of plasma volume using the dilution principle.

## Why Each Wrong Option is Incorrect
- **Option A:** is incorrect because, although it can be used to estimate blood volume, it also diffuses into tissues, making it less ideal for specifically measuring plasma volume.
- **Option B:** is incorrect because it rapidly diffuses out of the vascular space into the interstitial fluid, making it unsuitable for measuring plasma volume specifically.
- **Option C:** is incorrect because, similar to , it is not as confined to the vascular space as albumin and can distribute into other body compartments.

## Clinical Pearl / High-Yield Fact
A key point to remember is that albumin is primarily used to measure plasma volume due to its large size and confinement to the vascular space. This is crucial in clinical settings, such as assessing fluid status and guiding fluid management in critically ill patients.

## Correct Answer Line
**Correct Answer: D. Albumin**