Which etiology of Horner's syndrome is preganglionic in nature and typically presents with arm pain?

Horner's syndrome results from disruption of sympathetic nerve pathways to the eye and can have various etiologies. The answer that identifies the preganglionic nature of Horner's syndrome, particularly associated with arm pain, is linked to a Pancoast tumor.

A Pancoast tumor is a form of lung cancer located at the apex of the lung, which can invade nearby structures, including the brachial plexus and sympathetic chain. This tumor can cause a combination of symptoms due to its location: arm pain (often due to involvement of the brachial plexus), Horner's syndrome (due to disruption of sympathetics), and possibly other neurological deficits. The arm pain is attributable to the involvement of the roots of the brachial plexus, which while not a direct cause of Horner's syndrome, often accompanies the condition when the tumor is present at the apex of the lung.

The preganglionic nature is key to this diagnosis, as Pancoast tumors affect the sympathetic fibers originating from the first-order neurons in the lateral horn of the spinal cord that travel through the sympathetic chain before synapsing in the cervical ganglia. The other options typically involve different mechanisms or regions of nerve involvement that do not exclusively lead to