A tumor of the apex of the lung that is associated with Horner's syndrome is called what?

A tumor of the apex of the lung associated with Horner's syndrome is known as a Pancoast tumor. Pancoast tumors arise in the superior sulcus of the lung and have a distinctive characteristic of invading surrounding structures including nerves and brachial plexus, which can lead to specific symptoms. The involvement of sympathetic nerves, particularly the stellate ganglion, is what causes Horner's syndrome, which is characterized by ptosis (drooping of the eyelid), miosis (constricted pupil), and anhidrosis (lack of sweating) on the affected side of the face.

Mesothelioma, while a lung tumor, primarily originates from the pleura and does not typically present with Horner's syndrome. Carcinoid tumors, which are neuroendocrine tumors, are generally found in the lungs but are not located at the apex and do not commonly cause Horner's syndrome. Lung adenomas, benign tumors, also do not typically present in this manner and lack the specific relationship with the apex of the lung or the sympathetic nervous system that characterizes Pancoast tumors. Therefore, the unique association of a Pancoast tumor with Horner's syndrome is what establishes it as the correct answer.