**Question:** X-ray appearance of sequestrum is -

A. Increased density
B. Low density
C. Isolated area of increased density surrounded by low density
D. Absence of radiodensity

**Core Concept:**
Sequestrum is a term used in dentistry to describe a necrotic, non-vital portion of a tooth that is separated from the healthy pulp tissue. In an X-ray, sequestrum exhibits specific characteristics due to its composition and relationship to the surrounding tissues.

**Why the Correct Answer is Right:**
Correct Answer C (isolated area of increased density surrounded by low density) describes the X-ray appearance of sequestrum accurately. Sequestrum forms when a dead tooth part (necrosis) is separated from the healthy pulp tissue. In an X-ray, the necrotic tissue exhibits increased density due to calcification and mineralization. The low-density area surrounding the sequestrum represents the healthy tooth tissue and surrounding bone.

**Why Each Wrong Option is Incorrect:**
A. Increased density (Option A) inaccurately describes the entire sequestrum as having high density, rather than just the necrotic portion.
B. Low density (Option B) is the opposite of the correct description and does not reflect the increased density of the necrotic tissue.
D. Absence of radiodensity (Option D) is incorrect because sequestrum does exhibit increased density on X-ray, as explained above.

**Why the Correct Answer is Right:**
The correct answer, option C, is right because it accurately depicts the X-ray appearance of a sequestrum, which is composed of necrotic tissue with increased density surrounded by healthy tooth and surrounding bone with low density.

**Clinical Pearl:**
In clinical practice, understanding the X-ray appearance of sequestrum is crucial for diagnosing dental issues such as necrotic pulp tissue or periapical lesions. This knowledge aids in differentiating sequestrum from other dental pathologies, such as periapical granuloma or apical abscess, which may also present with a radiolucent area around a tooth, but have different radiological appearances.