## **Core Concept**
Ewing's sarcoma is a type of malignant bone tumor that primarily affects children and young adults. It is characterized by the presence of small, round, blue-staining cells under histology. These cells often have a high nuclear-to-cytoplasmic ratio and are typically filled with specific types of intermediate filaments.

## **Why the Correct Answer is Right**
The correct answer, **C. Intermediate filaments (specifically cytokeratin and vimentin, but most notably neurofilaments and especially desmin, but the hallmark is positivity for EWSR1-FLI1 fusion protein and usually contain glycogen)**, points towards the presence of glycogen in these cells. Ewing's sarcoma cells are known to contain abundant cytoplasmic glycogen, which can be demonstrated by Periodic Acid-Schiff (PAS) staining. This characteristic feature is often used in the diagnostic workup of Ewing's sarcoma.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because, although Ewing's sarcoma cells can have various organelles, they are not specifically known for being filled with lysosomes.
- **Option B:** This option is incorrect as Ewing's sarcoma cells are not particularly noted for containing large amounts of lipid droplets.
- **Option D:** This option is incorrect because, while the cells may have some rough endoplasmic reticulum, they are not characteristically filled with this organelle.

## **Clinical Pearl / High-Yield Fact**
A key diagnostic feature of Ewing's sarcoma is the presence of the **EWSR1-FLI1 fusion protein** in most cases, which results from a t(11;22) chromosomal translocation. This genetic abnormality is a hallmark of the disease and can be used as a diagnostic marker. Additionally, the **PAS positivity** due to glycogen accumulation is a helpful histological clue.

## **Correct Answer:** . Glycogen.