The mother of a one and a half year old child gives history of increasing proptosis of left eye for the past 1 month. Examination revels left eye white reflex. On computed tomography scan of the orbit there is left orbital mass with calcification mainly centred within the globe. The most likely diagnosis is?

**Core Concept**
The question is testing the ability to diagnose orbital masses in children, focusing on the **clinical presentation** and **imaging characteristics** of such masses. The key concept here involves understanding the typical features of various orbital tumors that can cause proptosis and leukocoria (white reflex) in a child.

**Why the Correct Answer is Right**
Given the child's age, symptoms, and the specific finding of an orbital mass with calcification centered within the globe on CT scan, the most likely diagnosis is a **retinoblastoma**. This is because retinoblastoma is a common intraocular malignancy in young children, often presenting with leukocoria and strabismus, and can cause proptosis if it extends beyond the globe. The presence of calcification within the tumor is a hallmark of retinoblastoma, making it distinguishable from other orbital masses.

**Why Each Wrong Option is Incorrect**
**Option A:** Would be incorrect because, although it could cause an orbital mass, the specific details given (especially calcification within the globe) do not align with this option.
**Option B:** and **Option C:** are also incorrect for similar reasons, as they do not typically present with the combination of leukocoria, proptosis, and intraocular calcification seen in retinoblastoma.

**Clinical Pearl / High-Yield Fact**
A critical point to remember is that **leukocoria** in a child, especially when accompanied by **strabismus** or **proptosis**, should prompt an urgent evaluation for retinoblastoma, given its potential for rapid progression and the importance of early treatment for preserving vision and life.

**Correct Answer:** D. Retinoblastoma.

✓ Correct Answer: B. Retinoblastoma