Retinoblastoma best evaluated by-
**Core Concept**
Retinoblastoma is a malignant tumor of the retina, primarily affecting young children. It arises from the retinal progenitor cells, which are sensitive to mutations in the RB1 gene. Understanding the genetic basis and early detection of retinoblastoma is crucial for effective management.
**Why the Correct Answer is Right**
The correct answer is **C. CT scan of the orbit**. A CT scan of the orbit is essential for evaluating retinoblastoma as it provides detailed images of the intraocular tumor, its extent, and any potential spread to surrounding tissues. The high-resolution images obtained from a CT scan enable accurate assessment of the tumor's size, shape, and location, which is critical for planning treatment.
**Why Each Wrong Option is Incorrect**
**Option A:** Ultrasound of the eye is not the best modality for evaluating retinoblastoma, as it may not provide sufficient detail about the tumor's extent and surrounding structures.
**Option B:** MRI of the orbit is not the primary imaging modality for retinoblastoma, although it may be used in certain situations to better visualize the tumor's relationship with surrounding structures.
**Option D:** Plain X-ray of the orbit is not sufficient for evaluating retinoblastoma, as it may not detect early tumors or provide detailed information about the tumor's extent.
**Clinical Pearl / High-Yield Fact**
It's essential to note that early detection of retinoblastoma is critical, and a thorough ophthalmological examination, including a dilated fundus examination, should be performed in any child presenting with leukocoria (white pupillary reflex) or strabismus.
**Correct Answer: C. CT scan of the orbit**