**Core Concept**
Pancoast's tumor is a type of lung cancer that arises from the apical segment of the upper lobe, typically involving the inferior surface of the lung. It is characterized by its proximity to the brachial plexus and other adjacent structures, leading to a unique set of clinical manifestations.

**Why the Correct Answer is Right**
Pancoast's tumor can cause Horner's syndrome due to the involvement of the sympathetic fibers in the stellate ganglion, resulting in ptosis, miosis, and anhidrosis. The tumor can also lead to atrophy of the deltoid muscle due to the compression of the suprascapular nerve. Radiotherapy and chemotherapy are common treatment modalities for Pancoast's tumor.

**Why Each Wrong Option is Incorrect**
**Option A:** Deltoid muscle atrophy is a true manifestation of Pancoast's tumor, making this option incorrect.
**Option B:** Radiotherapy and chemotherapy are indeed treatment options for Pancoast's tumor, so this option is also incorrect.
**Option C:** Horner's syndrome is a characteristic feature of Pancoast's tumor, making this option incorrect as well.

**Clinical Pearl / High-Yield Fact**
Pancoast's tumor is often associated with a classic triad of symptoms: pain at the apex of the lung, shoulder pain, and Horner's syndrome. This triad is a key clinical clue to the diagnosis of Pancoast's tumor.

**Correct Answer:** D. Pain along the ulnar border of the forearm