## Core Concept
The question tests knowledge of nuclear medicine, specifically the isotopes used for bone scans. Bone scans are a type of nuclear medicine imaging test that uses small amounts of radioactive material to visualize the bones. The most commonly used isotope for bone scans is one that accumulates in areas of high bone turnover.

## Why the Correct Answer is Right
Technetium-99m-methyl diphosphonate (Tc-99m MDP) is the correct answer because it is widely used for bone imaging. Tc-99m MDP accumulates in bone based on osteoblastic activity; areas of high bone turnover take up more of the tracer, which is then visualized using a gamma camera. This allows for the detection of various bone pathologies such as fractures, infections, and tumors.

## Why Each Wrong Option is Incorrect
* **Option A:** Iodine-131 is primarily used for thyroid imaging and therapy, not for bone scans. It targets the thyroid gland due to its affinity for iodine.
* **Option B:** Gallium-67 is used in infection and inflammation imaging, not primarily for bone scans. It accumulates in areas of inflammation.
* **Option D:** Thallium-201 is commonly used for myocardial perfusion imaging and certain types of tumor imaging, not for routine bone scans.

## Clinical Pearl / High-Yield Fact
A key point to remember is that **Tc-99m MDP** bone scans are highly sensitive for detecting areas of altered bone metabolism, making them useful for staging cancer, detecting stress fractures, and identifying areas of bone infection or inflammation.

## Correct Answer: C. Tc-99m MDP.