A 46-year-old man has had a slowly enlarging nodule on his trunk for the past 10 months. On examination, the 5-cm lesion is in the right anterior axillary line at the level of the nipple and has a central ulcerated plaque. The lesion is widely excised and on microscopic examination is composed of dense spindle cells in a storiform pattern that invades the dermal adipose tissue. The lesion recurs within a year, and the man is treated with a tyrosine kinase inhibitor. Molecular analysis of the lesion is most likely to show activation with overexpression of which of the following genes?

**Core Concept**
The underlying principle being tested involves the molecular pathogenesis of a specific type of skin tumor, likely a dermatofibrosarcoma protuberans (DFSP), given the description of the lesion and its behavior. This condition is characterized by a **storiform pattern** of dense spindle cells and a tendency to invade dermal adipose tissue.

**Why the Correct Answer is Right**
The clinical presentation and histological features suggest DFSP, a rare skin tumor known for its **recurrence rate** and potential for local invasion. DFSP is characterized by a translocation involving the **PDGFB** gene, which leads to its overexpression. This overexpression results in the activation of **tyrosine kinase receptors**, explaining the tumor's responsiveness to tyrosine kinase inhibitors.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because while other genes may be involved in various sarcomas, they are not specifically associated with DFSP.
**Option B:** Incorrect as it does not relate to the specific molecular anomaly seen in DFSP.
**Option C:** Incorrect because, although involved in some cancers, it is not the primary gene associated with DFSP.
**Option D:** Incorrect as the question context points towards a specific gene overexpression related to DFSP, not this option.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that DFSP is notable for its high recurrence rate after surgical excision and its potential responsiveness to **imatinib**, a tyrosine kinase inhibitor, due to the **PDGFB** gene's role in its pathogenesis.

**Correct Answer:** D. PDGFB

✓ Correct Answer: B. PDGFB