## **Core Concept**
The question revolves around the diagnosis of an undifferentiated tumor in the lung, which presents with symptoms like hemoptysis and cough in a 67-year-old male smoker. Undifferentiated tumors, also known as anaplastic or large cell carcinomas when specifically referring to lung cancer, lack specific features of differentiation, making their diagnosis and subtype classification challenging without immunohistochemistry.

## **Why the Correct Answer is Right**
The correct answer, **C. Cytokeratin**, is helpful because undifferentiated or large cell lung carcinomas are a type of non-small cell lung cancer (NSCLC). **Cytokeratins** are intermediate filament proteins found in the intracytoplasmic cytoskeleton of epithelial cells. Since carcinomas, including lung carcinomas, originate from epithelial cells, they typically express cytokeratins. Therefore, positivity for cytokeratins can help confirm the epithelial origin of an undifferentiated tumor, supporting the diagnosis of carcinoma, including lung cancer.

## **Why Each Wrong Option is Incorrect**
- **Option A: CD45** - This is a marker for leukocytes and is used to diagnose lymphomas or leukemic infiltrates. It would not be helpful in identifying carcinomas.
- **Option B: S-100** - While S-100 can be positive in various tumors, including some lung cancers like small cell carcinoma or metastatic melanomas, it is not specific for epithelial tumors and can be positive in a wide range of cell types.
- **Option D: Desmin** - This is a marker for muscle cells and is used to identify tumors of muscle origin, such as leiomyosarcomas or rhabdomyosarcomas. It would not be helpful in diagnosing epithelial tumors like lung carcinomas.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that in the context of lung tumors, especially undifferentiated ones, a panel of immunohistochemical markers is often used to narrow down the differential diagnosis. **Cytokeratin** positivity is crucial for confirming an epithelial origin, which supports a diagnosis of carcinoma. Smoking history, as in this case, increases the likelihood of lung cancer.

## **Correct Answer: C. Cytokeratin**