**Core Concept**
A ring sequestrum is a specific type of sequestrum, which is a piece of dead bone that has become separated from the living bone due to an infection or trauma. In the case of a ring sequestrum, the dead bone forms a ring-like structure around the affected area, typically as a result of osteomyelitis or a similar bone infection.

**Why the Correct Answer is Right**
A ring sequestrum is a characteristic feature of subacute or chronic osteomyelitis, where the infection has caused the formation of a sequestrum. The sequestrum acts as a barrier to the spread of infection, but also serves as a focus for further infection and inflammation. The ring-like shape of the sequestrum is due to the bone's attempt to wall off the infected area.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because ring sequestrum is not typically associated with acute osteomyelitis, which presents with more diffuse bone destruction and inflammation.
**Option B:** Incorrect because periostitis is inflammation of the periosteum, a layer of tissue surrounding the bone, and does not involve the formation of a sequestrum.
**Option C:** Incorrect because bone cysts are fluid-filled cavities within the bone, and do not involve the formation of a sequestrum.

**Clinical Pearl / High-Yield Fact**
A ring sequestrum is often seen in cases of subacute or chronic osteomyelitis, particularly in the long bones of the limbs. It is an important radiographic feature that can help diagnose this condition.

**Correct Answer: D.**