**Core Concept**
Radionuclide imaging, also known as nuclear medicine imaging, utilizes radiopharmaceuticals that localize to specific tissues or organs to produce images. In skeletal imaging, the radiopharmaceutical should have a high affinity for bone tissue and be able to provide information on bone structure and metabolism.

**Why the Correct Answer is Right**
The most useful radiopharmaceutical for skeletal imaging is Technetium-99m-methylene diphosphonate (Tc-99m-MDP). It localizes in bone through a process called adsorption, where the diphosphonate group binds to hydroxyapatite crystals in bone. This allows Tc-99m-MDP to visualize bone structure and identify areas of increased or decreased bone metabolism, such as fractures, tumors, or osteoporosis. The Technetium-99m radionuclide has a suitable half-life and energy for optimal imaging.

**Why Each Wrong Option is Incorrect**
* **Option A:** Technetium-99m-pentetate is a chelating agent used for renal imaging, not skeletal imaging.
* **Option B:** Gallium-67 citrate is used for imaging infections, tumors, and certain inflammatory conditions, but it is not specific for skeletal imaging.
* **Option C:** Indium-111 oxine is used for imaging infections, tumors, and certain inflammatory conditions, but it is not the most useful radiopharmaceutical for skeletal imaging.

**Clinical Pearl / High-Yield Fact**
Technetium-99m-MDP is a widely used radiopharmaceutical for skeletal imaging due to its high affinity for bone tissue and its ability to provide information on bone structure and metabolism. It is often used to diagnose and monitor conditions such as osteoporosis, bone fractures, and bone tumors.

**Correct Answer: C. Technetium-99m-methylene diphosphonate (Tc-99m-MDP)**