**Core Concept**
The halo sign refers to a characteristic appearance of cerebrospinal fluid (CSF) rhinorrhea on imaging studies, specifically on computed tomography (CT) scans. It is caused by the presence of a thin layer of blood surrounding a central layer of CSF, creating a distinct radiolucent halo.

**Why the Correct Answer is Right**
The halo sign occurs due to the flow of CSF through a dural defect or fistula, which is often accompanied by blood leakage. As the CSF flows, it pushes the blood forward, creating a layered appearance on imaging. This phenomenon is seen in cases of CSF rhinorrhea, which can be caused by trauma, surgery, or other conditions that compromise the integrity of the dura mater.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not relevant to the halo sign seen in CSF rhinorrhea.
**Option B:** This option is incorrect because it does not describe the underlying mechanism of the halo sign.
**Option C:** This option is not a correct description of the halo sign.

**Clinical Pearl / High-Yield Fact**
The halo sign is a crucial radiological finding that helps diagnose CSF rhinorrhea. It is essential to recognize this sign to avoid missing a potentially life-threatening condition.

**Correct Answer: D.**