**Core Concept**
A sequestrum is a piece of dead bone that has become separated from the living bone as a result of an infection or trauma. This occurs when the body's natural defense mechanisms attempt to isolate and wall off the infected or damaged bone tissue, leading to the formation of a sequestrum.

**Why the Correct Answer is Right**
The formation of a sequestrum is a result of the body's inflammatory response to an infection or injury. When the immune system detects the presence of pathogens or damaged tissue, it initiates an inflammatory response, which can lead to the formation of a sequestrum. This process involves the activation of various immune cells, including neutrophils and macrophages, which release enzymes and cytokines to break down the infected or damaged tissue. The resulting separation of the dead bone from the living bone tissue forms a sequestrum, which can then be isolated and removed through surgical intervention.

**Why Each Wrong Option is Incorrect**
**Option A:** A sequestrum is a type of bone tumor.
Incorrect because a sequestrum is not a tumor, but rather a piece of dead bone that has become separated from the living bone tissue.

**Option B:** A sequestrum is a type of bone graft.
Incorrect because a sequestrum is not a graft, but rather a piece of dead bone that has been isolated from the living bone tissue.

**Option C:** A sequestrum is a type of bone cyst.
Incorrect because a sequestrum is not a cyst, but rather a piece of dead bone that has become separated from the living bone tissue.

**Clinical Pearl / High-Yield Fact**
It's essential to recognize the signs and symptoms of osteomyelitis, including fever, pain, and swelling, as early diagnosis and treatment can prevent the formation of a sequestrum and subsequent bone damage.

**Correct Answer:** C. A piece of dead bone that has become separated from the living bone tissue.