**Question:** 7 years old child presents with a lesion in upper tibia. X-ray shows radiolucent area with Codman's triangle and Sunray appearance. Diagnosis is-

A. Osteoid osteoma
B. Osteosarcoma
C. Ewing's sarcoma
D. Haversian system

**Core Concept:**

The given clinical scenario involves a child presenting with a lesion in the upper tibia, characterized by a radiolucent area on X-ray and specific radiological appearances (Codman's triangle and Sunray appearance). These findings are indicative of a bone tumor.

**Why the Correct Answer is Right:**

The correct diagnosis among the given options is **Osteoid osteoma** (Option A). Osteoid osteoma is a benign bone tumor characterized by a small nidus (central area of osteoid deposition) surrounded by reactive bone. The radiological features mentioned (radiolucent area and specific appearances) are consistent with osteoid osteoma.

**Why Each Wrong Option is Incorrect:**

- **Osteosarcoma (Option B)** is a malignant bone tumor, which does not present with radiolucent areas and specific radiological appearances like Codman's triangle and Sunray appearance.
- **Ewing's sarcoma (Option C)** is also a malignant bone tumor; however, it usually affects younger patients and has distinct radiological features not matching the described findings.
- **Haversian system (Option D)** refers to the system of canals in bone tissue and is not a clinical entity related to bone tumors.

**Clinical Pearl:**

Recognizing the radiological appearances and appropriate age of presentation can help differentiate between benign (as in osteoid osteoma) and malignant (as in osteosarcoma) bone tumors. This knowledge is crucial for selecting the correct management strategy and providing appropriate treatment to the patient.