A 67 year male smoker presents with haemoptysis and cough. Bronchoscopic biopsy revealed undifferentiated tumour. The immunohistochemical marker that can be most helpful is:

Immunohistochemistry is used in conjunction with light microscopy to distinguish between primary and metastatic adenocarcinoma, determine neuroendocrine features, diagnose mesotheliomas. Pulmonary adenocarcinomas usually stain positive for cytokeratin 7, thyroid transcription factor 1 (TTF-1), and surfactant apoprotein A and are negative for cytokeratin 20. Metastatic adenocarcinomas from other sites except the thyroid stain negative for TTF-1. All carcinoids and most SCLCs stain positive for chromogranin and synaptophysin, whereas NSCLC is usually negative for these two markers. Mesothelioma is distinguished from adenocarcinoma by the presence of calretinin and cytokeratin 5/6 and the absence of carcinoembryonic antigen (CEA), B72.3, Ber-EP4, and MOC-31. Ref: Gibbons D., Pisters K.M., Johnson F., Eapen G.A. (2011). Chapter 15. Non-Small Cell Lung Cancer. In H.M. Kantarjian, R.A. Wolff, C.A. Koller (Eds), The MD Anderson Manual of Medical Oncology, 2e.