**Core Concept:** Metastasis is the process by which cancer cells from a primary tumor spread to other parts of the body, often causing life-threatening complications. In children, the most common site for tumor spread is the orbit, a bony cavity containing the eye and its surrounding structures.

**Why the Correct Answer is Right:** The correct answer indicates that rhabdomyosarcoma (RMS) is the most common tumor to spread to the orbit in children. RMS is a soft tissue sarcoma that originates from embryonal or alveolar myoblasts and typically occurs in the head and neck region. When RMS spreads to the orbit, it often presents with eye symptoms like proptosis, diplopia, and visual disturbances.

**Why Each Wrong Option is Incorrect:**

A. Wilms Tumor (nephroblastoma): This tumor primarily affects the kidneys and does not commonly spread to the orbit.
B. Medulloblastoma: This is a brain tumor that rarely spreads to the orbit.
C. Ewing's sarcoma: This is a bone tumor that rarely spreads to the orbit.
D. Retinoblastoma: This is an eye tumor, but it originates from the retina, not the orbit, and causes different symptoms.

**Clinical Pearl:** The correct answer, Rhabdomyosarcoma, is worth remembering because it is the most common pediatric orbital tumor and can cause significant morbidity if not diagnosed and treated promptly. Early recognition and timely intervention are crucial to improve prognosis and patient outcomes.