While large cell lung carcinoma is a possibility in a patient with a heavy smoking history, it does not classically present with a Pancoast tumor.

A lung abscess causes very different symptoms, typically fever, sweats, cough, and expectoration of foul-smelling sputum. It is not associated with Horner's syndrome.

The patient's signs and symptoms suggest a Pancoast tumor, which is most often caused by non-small cell lung carcinomas, primarily adenocarcinoma and squamous cell carcinoma. Among them, adenocarcinoma is the most common. Pancoast tumors are associated with a triad of shoulder pain, arm weakness, and Horner's syndrome (ptosis, miosis, anhidrosis), caused by tumor invasion into the brachial plexus and sympathetic chain.

While lung cancers can metastasize to the brain, they would likely cause neurologic signs and symptoms localized to the affected brain region. Horner's syndrome is more likely due to local invasion of the tumor into the sympathetic chain at the apex of the lung.