**Question:** All are true about aneurysmal bone cyst except -

A. It is a benign tumor
B. It is a malignant tumor
C. It is a congenital lesion
D. It is a reactive lesion

**Core Concept:**
Aneurysmal bone cyst (ABC) is a benign, vascular lesion characterized by a cystic expansile mass in the bone. These cysts are lined by a single layer of flattened, thin-walled endothelial cells and communicate with the bone marrow. ABCs are classified into primary and secondary types. Primary ABCs occur de novo, while secondary ABCs are associated with underlying bone lesions like benign or malignant tumors, trauma, or fractures.

**Why the Correct Answer is Right:**
Option B ("It is a malignant tumor") is incorrect because aneurysmal bone cyst is a benign (non-cancerous) lesion. Benign tumors grow slowly, do not invade surrounding tissues, and are not life-threatening. In contrast, malignant tumors (also known as cancers) have aggressive growth, can invade surrounding tissues, and can lead to life-threatening complications.

**Why Each Wrong Option is Incorrect:**
Option A ("It is a benign tumor") is correct as stated above. Aneurysmal bone cyst is a benign lesion, meaning it is slow-growing and does not invade surrounding tissues.

Option C ("It is a congenital lesion") is incorrect as aneurysmal bone cyst can occur at any age and is not exclusively present at birth (congenital). While they might be present since birth, the term "congenital" specifically refers to lesions present at birth, making this option incorrect.

Option D ("It is a reactive lesion") is also correct as aneurysmal bone cyst is considered a reactive lesion due to its association with underlying bone lesions or trauma. Reactive lesions are caused by an exaggerated response to an external stimulus (in this case, underlying bone lesions or trauma) rather than being the primary cause itself.

**Clinical Pearl:**
Understanding the classification of tumors (benign vs. malignant) is crucial in clinical practice. Benign lesions are not life-threatening, while malignant lesions can pose significant risks to the patient's health and can spread to other parts of the body. It is essential to identify the type of tumor to formulate an appropriate treatment plan and monitor the patient's prognosis.