**Core Concept**
The question requires the application of immunohistochemistry (IHC) markers to differentiate a centrally located lung mass in a patient with a history of chronic smoking and hemoptysis. The correct IHC marker should help identify the cell of origin for the undifferentiated tumor.

**Why the Correct Answer is Right**
Cytokeratin is a family of intermediate filament proteins that are specific to epithelial cells. In the context of lung pathology, cytokeratin IHC is useful for identifying carcinomas, which are the most common type of lung tumor. Centrally located lung masses are more likely to be carcinomas, and cytokeratin staining can help confirm this diagnosis. The presence of cytokeratin in a biopsy specimen suggests that the tumor cells are epithelial in origin.

**Why Each Wrong Option is Incorrect**
**Option B:** Parvalbumin is a calcium-binding protein found in muscle cells, particularly in fast-twitch muscle fibers. It is not typically used as an IHC marker in lung pathology and would not be useful in identifying the cell of origin for a lung tumor.

**Option C:** HMB-45 is a monoclonal antibody that reacts with melanocytic tumors, such as melanoma. While melanoma can occur in the lung, it is not the most likely diagnosis for a centrally located lung mass in a patient with a history of chronic smoking.

**Option D:** Hep-par1 is a monoclonal antibody that reacts with hepatocellular carcinoma, a primary liver cancer. It is not typically used as an IHC marker in lung pathology and would not be useful in identifying the cell of origin for a lung tumor.

**Clinical Pearl / High-Yield Fact**
When faced with an undifferentiated tumor, it is essential to use a panel of IHC markers to narrow down the differential diagnosis. In this case, cytokeratin is a useful starting point, as it can help identify carcinomas, which are the most common type of lung tumor.

**Correct Answer Line**
✓ Correct Answer: A. Cytokeratin.