**Core Concept**
Ely's cysts are a type of benign lesion that occurs due to the entrapment of the epithelial lining of the tendon sheath, leading to a fluid-filled cystic structure. This occurs due to the repetitive trauma and inflammation of the tendon sheath, resulting in the formation of a cystic space.

**Why the Correct Answer is Right**
The correct answer is related to the pathology of Ely's cysts, which involves the entrapment of the epithelial lining of the tendon sheath. This is a classic example of a ganglion cyst, which is a type of cyst that forms in the tendon sheath due to the accumulation of fluid. The entrapment of the epithelial lining leads to the formation of a cystic space, which can cause symptoms such as pain and swelling.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because Ely's cysts are not a feature of osteochondritis dissecans. Osteochondritis dissecans is a condition that affects the joint and involves the separation of a piece of cartilage and bone from the rest of the joint.
* **Option B:** This option is incorrect because Ely's cysts are not a feature of bursitis. Bursitis is a condition that affects the bursae, which are fluid-filled sacs that cushion the joints and reduce friction between moving parts.
* **Option C:** This option is incorrect because Ely's cysts are not a feature of tenosynovitis. Tenosynovitis is a condition that affects the tendon sheath and involves inflammation of the tendon sheath.

**Clinical Pearl / High-Yield Fact**
Ely's cysts are also known as ganglion cysts and are commonly found on the wrist and foot. They are usually benign and can be treated with aspiration or surgical removal.

**Correct Answer: C. Ganglion cyst.