Final answer:
Horner's syndrome can be caused by a tumor in the thoracic cavity.
Step-by-step explanation:
Horner's syndrome is a condition characterized by a set of symptoms that affect one eye, including constricted pupil and drooping of the eyelid. It is caused by damage or disruption of the sympathetic nerve pathway that controls these functions. When a tumor develops in the thoracic cavity, it may compress or damage the sympathetic nerves, leading to the development of Horner's syndrome. So, if Horner's syndrome arises between the ages of 20-50 years old, one should suspect a tumor in the thoracic cavity as the underlying cause.