A 56 year old Chronic smoker, mass in bronchus resected. Most useful immunohistochemical marker to make a proper diagnosis would be -
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.
Cytokeratin
Vimentin
Epithelial membrane cadherin
Leucocyte common antigen