**Core Concept**
The question tests the ability to identify a specific radiological pattern associated with a particular condition, specifically a solitary collapsed dorsal vertebra with preserved disc spaces. This pattern is characteristic of certain types of bone lesions that can affect children.

**Why the Correct Answer is Right**
Histiocytosis, specifically Langerhans cell histiocytosis (LCH), is a condition that can cause a solitary lytic lesion in the vertebral body, leading to vertebral collapse. This occurs due to the infiltration of Langerhans cells into the bone marrow, which can cause destruction of the vertebral body. The disc spaces are preserved because the disc is not directly affected by the histiocytic infiltration. The absence of an associated soft tissue shadow is also consistent with LCH, as it often presents as a solitary bony lesion without significant soft tissue involvement.

**Why Each Wrong Option is Incorrect**
**Option A:** Ewing's sarcoma is a malignant bone tumor that typically presents with a mixed lytic and blastic lesion, often with associated soft tissue mass. It is less likely to cause a solitary collapsed vertebra with preserved disc spaces.
**Option B:** Tuberculosis of the spine, also known as Pott's disease, typically presents with multiple levels of vertebral body destruction, disc space narrowing, and associated soft tissue shadow. This is in contrast to the solitary lesion seen in LCH.
**Option D:** Metastasis to the spine can occur from various primary tumors, but it is less common in children and typically presents with multiple lesions, often with associated soft tissue mass.

**Clinical Pearl / High-Yield Fact**
When evaluating a child with a solitary collapsed vertebra and preserved disc spaces, Langerhans cell histiocytosis should be considered in the differential diagnosis. This condition can present with a wide range of systemic symptoms, including fever, weight loss, and bone pain, in addition to the characteristic radiological findings.

✓ Correct Answer: C. Histiocytosis