**Core Concept**
Orbital metastasis in children is a serious condition where a tumor from another part of the body spreads to the orbit, affecting the eye and surrounding tissues. This can result from various primary malignancies, with the most common ones being neuroblastoma, lymphoma, and leukemia.

**Why the Correct Answer is Right**
The most common metastatic orbital tumor in children is neuroblastoma. Neuroblastoma originates from the adrenal glands but can also arise from the sympathetic nervous system, including the orbit. When it metastasizes to the orbit, it can cause proptosis, eyelid swelling, and vision loss due to compression of the optic nerve. The primary mechanism involves the spread of neuroblastoma cells through the bloodstream, which can lodge in the orbital tissues and establish a metastatic tumor.

**Why Each Wrong Option is Incorrect**
* **Option A:** Although lymphoma can metastasize to the orbit, it is less common in children compared to neuroblastoma.
* **Option B:** Leukemia can cause orbital involvement, but this is typically a secondary effect due to bone marrow infiltration, rather than a primary metastatic tumor.
* **Option C:** Rhabdomyosarcoma is a primary orbital tumor, but it is not typically metastatic in nature, whereas neuroblastoma is a metastatic tumor to the orbit.

**Clinical Pearl / High-Yield Fact**
Neuroblastoma metastasis to the orbit is often associated with the presence of opsoclonus-myoclonus syndrome, a paraneoplastic condition characterized by abnormal eye movements and muscle spasms.

**Correct Answer:** D. Neuroblastoma