**Question:** Radiographically 'Sequestrum' formed in osteomyelitis is:

A. A radiolucent zone
B. A radiopaque zone
C. A radiolucent zone with a surrounding radiopaque halo
D. A radiolucent zone without a surrounding halo

**Correct Answer:** D. A radiolucent zone without a surrounding halo

**Core Concept:**

Osteomyelitis is a bone infection caused by bacterial invasion of the bone marrow. When an osteomyelitis progresses, it may lead to the formation of a sequestrum. A sequestrum is a portion of bone that has become avascular due to the destruction of surrounding soft tissue and blood vessels by the infection. This leads to a decrease in calcium deposition, making the sequestrum radiolucent when viewed on X-rays or radiographs.

**Why the Correct Answer is Right:**

The correct answer, D, explains that a sequestrum appears as a radiolucent zone on radiographs. This is because the sequestrum lacks blood supply and consequently contains less calcium, making it radiolucent.

**Why Each Wrong Option is Incorrect:**

A and B are incorrect because sequestra are radiolucent, not radiopaque zones.

C is incorrect because a sequestrum has a surrounding radiopaque halo due to the surrounding healthy bone tissue.

**Clinical Pearl:**

Understanding the radiographic appearance of sequestra is crucial for diagnosing osteomyelitis. In clinical practice, recognizing sequestra can help differentiate between an active osteomyelitis and a healed infection, as a sequestrum is typically an indicator of an active infection. Additionally, it is essential to differentiate sequestra from other radiolucent bone lesions, such as those caused by tumors, fractures, or avascular necrosis, to avoid misdiagnosis and unnecessary further investigations or treatments.