**Core Concept**
Childhood malignancies can cause proptosis due to orbital involvement or metastasis. Proptosis refers to the forward displacement of the globe, which can be caused by various factors including tumors, inflammatory conditions, and vascular anomalies.

**Why the Correct Answer is Right**
Rhabdomyosarcoma and neuroblastoma are two common childhood malignancies that can cause proptosis. Rhabdomyosarcoma is the most common primary orbital malignancy in children, often presenting with proptosis and eyelid swelling. Neuroblastoma, a type of extracranial solid tumor, can metastasize to the orbit and cause proptosis, particularly in infants and young children. The mechanism involves the infiltration of tumor cells into the orbital tissues, leading to displacement of the globe.

**Why Each Wrong Option is Incorrect**
* **Option A:** Although lymphoma is a malignancy, it is less common in children and typically presents with systemic symptoms rather than isolated proptosis.
* **Option B:** Osteosarcoma is a primary bone malignancy that can involve the orbit, but it is more common in older children and adolescents.
* **Option C:** Leukemia can cause orbital involvement, but it typically presents with signs of bone marrow failure, such as anemia, thrombocytopenia, and infections.

**Clinical Pearl / High-Yield Fact**
Rhabdomyosarcoma and neuroblastoma are the two most common childhood malignancies causing proptosis, and they often present with distinct clinical features. Rhabdomyosarcoma typically involves the superolateral quadrant of the orbit, while neuroblastoma often involves the inferior and medial quadrants.

**Correct Answer: C. Neuroblastoma**