**Question:** A 55 year-old man from Chicago complains of pain in his left thigh for five months. On physical examination his thigh is increased in size, compared to the right. A plain film radiograph reveals the presence of a 15 cm solid mass that does not appear to arise from bone, but it does have infiltrative margins. A biopsy of this mass is taken, and on microscopic examination the mass is composed of highly pleomorphic spindle cells. Which of the following markers is most likely to be demonstrated in the cells of this mass?

A. S-100
B. Desmin
C. Vimentin
D. GFAP

**Core Concept:** Spindle cell tumors are a diverse group of neoplasms characterized by the presence of spindle-shaped cells. These tumors can be benign or malignant, and are derived from various cell lineages, including smooth muscle, nerve sheath, and mesenchymal cells. Spindle cells express various markers depending on their lineage, helping in their classification and diagnosis.

**Why the Correct Answer is C (Vimentin):**

Vimentin is a type III intermediate filament protein that is expressed in mesenchymal cells, which includes cells originating from connective tissue, muscle, and bone. In the context of this question, vimentin is most likely to be demonstrated in the cells of a mesenchymal tumor, such as the one described.

**Why Each Wrong Answer is Incorrect:**

A. S-100: This marker is primarily expressed in Schwann cells, which are part of the peripheral nervous system, and not in mesenchymal cells.
B. Desmin: This marker is expressed in smooth muscle cells, not mesenchymal cells.
D. GFAP (Glial Fibrillary Acidic Protein): This marker is expressed in astrocytes of the central nervous system, again not in mesenchymal cells.

**Clinical Pearls:**

Understanding the expression of these markers helps in differentiating between various types of spindle cell tumors, and can have a significant impact on their diagnosis and treatment. For example, a positive staining for S-100 would suggest a Schwann cell tumor, while a positive staining for GFAP would suggest a central nervous system tumor.

**Core Concept:** Spindle cell tumors can be classified based on their lineage and markers expressed. These markers are essential for accurate diagnosis and prognosis.

**Why the Correct Answer is C (Vimentin):**

Vimentin is expressed in mesenchymal cells, including those of the connective tissue, muscle, and bone. In this context, vimentin would be positive in a mesenchymal tumor, like the one described in the question.

**Why Each Wrong Answer is Incorrect:**

A. S-100: This marker is expressed in Schwann cells, which are part of the peripheral nervous system, not mesenchymal cells.
B. Desmin: This marker is expressed in smooth muscle cells, not mesenchymal cells.
D. GFAP (Glial Fibrillary Acidic Protein): This marker is expressed in astrocytes of the