Okay, let's tackle this question. The patient is a 23-year-old with leucocoria in the right eye for two months, and there's total retinal detachment. The ultrasound shows a heterogeneous subretinal mass with calcification. I need to figure out the most likely diagnosis.

First, leucocoria is a white pupil reflex, which is a red flag for retinal issues. Common causes include retinoblastoma, Coats disease, retinal detachment with a mass, or other tumors. Since there's a subretinal mass with calcification, retinoblastoma comes to mind. Retinoblastoma is a pediatric cancer, typically under 5 years, but can present in older kids. It's known for calcifications and causing retinal detachment.

But wait, the patient is 23. Retinoblastoma is more common in children under 5. What about Coats disease? That's more in males around 10-20 years old. It causes telangiectatic vessels and exudation, leading to retinal detachment. But does it have calcifications? Not typically. Maybe a retinal astrocytoma? Those are benign and can calcify, but they're more associated with tuberous sclerosis.

Another possibility is a choroidal melanoma. But those are more in adults and usually don't cause leucocoria. They might have calcifications but are subretinal masses. However, retinal detachment is less common with melanoma.

Wait, the ultrasound shows a heterogeneous subretinal mass with calcification. Retinoblastoma is the most likely here even in a 23-year-old, though it's rare. Maybe the tumor has persisted or is a late presentation. Alternatively, could it be a metastasis? Unlikely in a young person without a primary cancer history.

So the correct answer is retinoblastoma. The other options don't fit as well. Coats lacks calcification, melanoma is less likely in this age, and astrocytoma is associated with different syndromes. The key points are leucocoria, subretinal mass with calcification, and retinal detachment pointing to retinoblastoma.

**Core Concept**
This question tests recognition of **retinoblastoma**, a pediatric intraocular malignancy characterized by **leucocoria**, **retinal detachment**, and **calcified subretinal masses** on imaging. It is the most common primary intraocular malignancy in children but can rarely present in older individuals.

**Why the Correct Answer is Right**
Retinoblastoma arises from retinal progenitor cells and typically presents with **leucocoria** (white pupil reflex) due to tumor excavation of the retina. The **calcified subretinal mass** on ultrasound is pathognomonic, as retinoblastoma frequently exhibits calcifications due to tumor necrosis and mineralization. Retinal detachment occurs secondary to tumor growth and exudative retinal separation. While most cases are diagnosed in children under 5 years, sporadic adult-onset cases can occur, especially if the tumor was undiagnosed in childhood.

**Why Each Wrong Option is Incorrect**
**Option A:** *Coats disease* is a unilateral retinal telangiectasia with lipid exudation, but

✓ Correct Answer: B. Retinoblastoma