**Question:** Wide diploic space of skull with brush border (hair on end) appearance is characteristic of

**Core Concept:** The diploic space is a cavum within the calvarium which contains the diploë, a spongy bone made up of trabecular bone with a layer of compact bone on both sides. The brush border appearance is a term used to describe the irregular, hair-like pattern seen on imaging studies.

**Why the Correct Answer is Right:** In the context of the question, the brush border appearance in the diploic space indicates expansion of the diploë, which is typically seen in conditions causing increased intracranial pressure (ICP). This expansion is due to the stretching of the diploë and the adjacent bone marrow spaces, resulting in a "hair on end" appearance.

**Why Each Wrong Option is Incorrect:**

A. **Increased bone density (osteoporosis)**: Osteoporosis results in a decrease in bone density and can cause decreased bone thickness, not expansion.

B. **Normal bone marrow**: The brush border appearance is attributed to increased bone density and not normal bone marrow.

C. **Increased bone thickness**: The question indicates expansion, not thickening, so this option is incorrect.

D. **Bone infection (osteomyelitis)**: Although infection can cause bone expansion, the specific term "brush border" is associated with increased bone density and not infection.

**Clinical Pearl / High-Yield Fact:** The presence of a brush border appearance in the diploic space, along with other imaging findings, can help in diagnosing conditions like increased ICP, such as subdural or epidural hematomas, subarachnoid hemorrhages, or brain tumors. However, it is essential to consider other imaging findings and clinical context to make the correct diagnosis.

**Correct Answer:** D. Increased bone density (osteoporosis)