**Core Concept**
The "hair-on-end" appearance is a radiological term used to describe a specific pattern of increased uptake on bone scans, typically seen in conditions where there is an increase in osteoblastic activity. This appearance is characterized by a "hair-on-end" or "sunburst" pattern of increased uptake, often seen in conditions such as osteosarcoma, Brodie's abscess, and other bone tumors.

**Why the Correct Answer is Right**
The hair-on-end appearance is caused by an increase in osteoblastic activity, which is the process of bone formation. This is often seen in conditions where there is a high turnover of bone, such as in osteosarcoma, where there is a rapid growth of new bone tissue. The increased osteoblastic activity leads to an increase in the uptake of the radiotracer, resulting in the characteristic hair-on-end appearance on bone scans.

**Why Each Wrong Option is Incorrect**
**Option A:** Osteoporosis is incorrect because it is characterized by a decrease in bone density and an increase in osteoclastic activity, which would lead to a decrease in radiotracer uptake, not an increase.

**Option B:** Aneurysmal bone cyst is incorrect because it is a type of benign bone tumor that can cause a hair-on-end appearance due to its high turnover of bone tissue.

**Option C:** Fibrous dysplasia is incorrect because it is a type of bone disorder that can cause a variety of radiological abnormalities, but it is not typically associated with a hair-on-end appearance.

**Option D:** Osteoblastoma is incorrect because it is a type of benign bone tumor that can cause a hair-on-end appearance due to its high osteoblastic activity.

**Clinical Pearl / High-Yield Fact**
A hair-on-end appearance on a bone scan can be a classic sign of a malignant bone tumor, such as osteosarcoma. It is essential to consider this appearance in the context of the patient's clinical presentation and other radiological findings.

**Correct Answer:** C.