**Core Concept**
Rib notching on X-ray is a radiological sign that indicates erosion of the lower margin of the ribs, usually due to the erosion of the costovertebral joints. This phenomenon is often associated with vascular or neoplastic conditions that cause external compression or erosion of the ribs.

**Why the Correct Answer is Right**
The correct answer is related to the compression of intercostal vessels (arteries and veins) by a tumor or an aneurysm of the aorta (also known as aortic compression syndrome). This compression leads to the erosion of the ribs due to the chronic ischemia of the intercostal muscles. The most common cause of rib notching in adults is the compression of intercostal arteries by a thoracic aortic aneurysm or coarctation of the aorta.

**Why Each Wrong Option is Incorrect**
* **Option A:** This is not related to rib notching. Fibrous dysplasia is a condition characterized by fibrous tissue replacing normal bone, but it does not typically cause rib notching.
* **Option B:** This option is incorrect because osteoporosis is a condition that weakens bones, but it does not specifically cause rib notching. Osteoporosis can lead to compression fractures, but it is not directly associated with rib notching.
* **Option D:** This option is not related to rib notching. Sarcoidosis is a systemic granulomatous disease that can affect multiple organs, including the lungs and lymph nodes, but it is not typically associated with rib notching.

**Clinical Pearl / High-Yield Fact**
Remember that rib notching can be caused by a variety of conditions, including vascular, neoplastic, and infectious processes. A thorough history and physical examination, combined with radiographic findings, are essential for making an accurate diagnosis.

**Correct Answer:** C.