## Core Concept
The Ploucquet test is a method used to determine if a fetus was live-born. It involves measuring the **specific gravity** of the lungs to assess if they have taken in air, which is indicative of breathing and thus, life.

## Why the Correct Answer is Right
The Ploucquet test specifically measures the **specific gravity of the lungs**. In a live-born fetus, the lungs will have taken in air and therefore have a lower specific gravity compared to a stillborn fetus, whose lungs would have a higher specific gravity similar to that of the liver. This test helps in determining if a baby was born alive or not, which can be crucial in legal and medical investigations.

## Why Each Wrong Option is Incorrect
* **Option A:** This option is incorrect because while the test does involve a physical measurement, it specifically targets the lungs' characteristics post-mortem to infer live birth.
* **Option B:** This option is incorrect as there isn't a widely recognized association of another bodily fluid's measurement directly linked to the Ploucquet test's purpose.
* **Option C:** This option is incorrect because, although liver and spleen conditions can be relevant in forensic pathology, the Ploucquet test specifically focuses on lung characteristics.
* **Option D:** This option is incorrect because, similar to option C, while other organ conditions are relevant in pathology, the Ploucquet test is specifically about lung changes indicative of air intake.

## Clinical Pearl / High-Yield Fact
A key point to remember is that the Ploucquet test, while historically significant, has largely been replaced by more modern and accurate methods for determining live birth. However, understanding its basis on lung density changes helps in grasping forensic pathology principles.

## Correct Answer: D. Lung.