A 56 year old Chronic smoker, mass in bronchus resected. Most useful immunohistochemical marker to make a proper diagnosis would be -
Cytokeratin
Vimentin
Epithelial membrane cadherin
Leucocyte common antigen
Mass in bronchus in an old chronic smoker suggest bronchogenic carcinoma. The marker of carcinoma, on immunohistochemistry, is cytokeratin.
Other epithelial markers for carcinoma are :-


Epithelial membrane antigen (EMA):- Breast, lung and Kidney's adenocarcinoma.
Carcino-embryonic antigen: - Carcinomas of GIT, pancreas, lung, breast, uterus and ovary.