## Core Concept
The "onion-peel" appearance is a radiological finding typically associated with certain bone lesions. This appearance is characterized by layers of reactive bone formation around a lesion, resembling the layers of an onion. It is often seen in conditions that cause periosteal reaction.

## Why the Correct Answer is Right
The correct answer, **C. Ewing's sarcoma**, is associated with an "onion-peel" appearance due to the characteristic periosteal reaction it causes. Ewing's sarcoma is a malignant bone tumor that often presents in children and adolescents. The periosteum, a layer of connective tissue covering the bone, reacts to the tumor by forming layers of new bone, which on radiographs, appear as concentric, lamellated, or "onion-skin" layers. This pattern is a result of the tumor's rapid growth, which lifts the periosteum off the bone, leading to intermittent periods of bone formation.

## Why Each Wrong Option is Incorrect
- **Option A:** While osteomyelitis can cause periosteal reaction, it more commonly results in a periosteal new bone formation that appears as a single layer or a thick, irregular layer rather than the characteristic "onion-peel" appearance.
- **Option B:** Osteosarcoma (osteogenic sarcoma) can also cause a periosteal reaction, but it typically results in a Codman's triangle or a "sun-ray" appearance rather than an "onion-peel" appearance.
- **Option D:** A bone cyst (simple or unicameral bone cyst) is usually a benign, fluid-filled lesion that can cause a pathological fracture but does not typically produce an "onion-peel" appearance.

## Clinical Pearl / High-Yield Fact
A key clinical pearl is that the "onion-peel" appearance is highly suggestive of Ewing's sarcoma, particularly in the context of a young patient with a bone tumor. However, it's essential to differentiate it from other conditions that can cause similar radiographic findings.

## Correct Answer: C. Ewing's sarcoma.