**Core Concept**
The use of Technetium-99m (**Tc-99m**) labelled Red Blood Cells (**RBCs**) is a diagnostic tool in nuclear medicine, primarily for assessing **cardiovascular** and **hematological** conditions. This technique exploits the properties of **Tc-99m** to visualize and quantify **RBC** distribution and movement within the body.

**Why the Correct Answer is Right**
Given the common applications of **Tc-99m** labelled **RBCs**, they are particularly useful for imaging the **heart** and **lungs**, allowing for the evaluation of **cardiac function** and the detection of **gastrointestinal bleeding**. The labelling process involves tagging **RBCs** with **Tc-99m**, which then circulate and can be traced using **gamma camera** imaging.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because, while **Tc-99m** is used in various imaging studies, the specific application of labelled **RBCs** is more targeted.
**Option B:** Incorrect as it does not directly relate to the primary use of **Tc-99m** labelled **RBCs**.
**Option C:** Incorrect because, although **Tc-99m** has a wide range of applications, the use of labelled **RBCs** is specific to certain diagnostic needs.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that **Tc-99m** labelled **RBCs** are particularly useful for detecting **gastrointestinal bleeding** due to their ability to accurately localize the site of bleeding, guiding potential interventions.

**Correct Answer:** D. Gastrointestinal bleeding