**Core Concept**
The question requires the identification of a specific immunohistochemical marker useful for diagnosing a mass in the bronchus, particularly in a 56-year-old chronic smoker. The correct marker should aid in distinguishing between various types of lung tumors, taking into account the patient's smoking history.

**Why the Correct Answer is Right**
In the context of lung cancer, particularly in smokers, the most useful immunohistochemical marker is TTF-1 (Thyroid Transcription Factor-1). TTF-1 is a nuclear protein expressed in the thyroid gland and certain types of lung cancer, including adenocarcinoma and squamous cell carcinoma. It is essential for diagnosing and subclassifying non-small cell lung carcinomas (NSCLCs). TTF-1 positivity helps differentiate adenocarcinoma from other types of lung cancer, such as small cell carcinoma and carcinoids.

**Why Each Wrong Option is Incorrect**
* **Option A:** While p63 is a useful marker for squamous cell carcinoma, it is not as specific or sensitive as TTF-1 for diagnosing lung cancer in smokers.
* **Option B:** CK7 is a useful marker for distinguishing adenocarcinoma from squamous cell carcinoma, but it is not as specific as TTF-1 for diagnosing lung cancer in smokers.
* **Option C:** Napsin A is a useful marker for adenocarcinoma, but it is not as sensitive as TTF-1 for diagnosing lung cancer in smokers.

**Clinical Pearl / High-Yield Fact**
Remember that the choice of immunohistochemical marker depends on the clinical context, including the patient's smoking history and the type of lung cancer suspected. TTF-1 is a valuable marker for diagnosing and subclassifying NSCLCs, particularly in smokers.

**Correct Answer:** C. Napsin A