## Core Concept
Technetium-99m (Tc-99m) labelled red blood cells (RBCs) are primarily used in nuclear medicine imaging to visualize and assess the structure and function of the heart, particularly for detecting myocardial perfusion and viability. This technique takes advantage of the ability of Tc-99m to bind to RBCs, allowing for the tracking of cardiac function.

## Why the Correct Answer is Right
The correct use of Tc labelled RBC's is in **Gastrointestinal Bleeding**, specifically for **identifying and localizing the site of gastrointestinal bleeding**. When Tc-99m labelled RBCs are reinjected into the patient, they circulate and can be detected by a gamma camera. Areas of gastrointestinal bleeding will accumulate these labelled cells, which can then be visualized. This method is particularly useful because it allows for the detection of active bleeding sites that may not be evident through other imaging modalities.

## Why Each Wrong Option is Incorrect
- **Option A:** While some nuclear medicine procedures involve the liver and spleen, Tc labelled RBCs are not primarily used for imaging these organs; other agents like Tc-99m sulphur colloid are more commonly used for liver and spleen imaging.
- **Option B:** This option is not specified, but based on the context, it's likely related to cardiac imaging. Although Tc-99m labelled RBCs can be used for cardiac imaging (e.g., multiple-gated acquisition scan or MUGA scan), the question seems to point towards a more specific application.
- **Option C:** Similarly, this option is unspecified but could relate to various imaging procedures. The specificity of Tc labelled RBCs for gastrointestinal bleeding makes it a standout application.

## Clinical Pearl / High-Yield Fact
A key clinical pearl is that **Tc-99m labelled RBCs have a longer half-life in circulation compared to other labelling methods**, making them suitable for studies that require longer imaging times, such as detecting intermittent gastrointestinal bleeding. This property allows for dynamic imaging over a period, increasing the chances of capturing the bleeding site.

## Correct Answer Line
**Correct Answer: D. Gastrointestinal bleeding.**