**Core Concept**
Squamous cell carcinoma (SCC) of the lung is a type of non-small cell lung cancer (NSCLC) that arises from the epithelial cells of the bronchial tree. It is often associated with smoking and tends to arise in the central airways, particularly in the upper lobes of the lungs.

**Why the Correct Answer is Right**
The clinical presentation described in the question is suggestive of Horner's syndrome, which is characterized by an ipsilateral drooping upper eyelid (ptosis), decreased pupil size (miosis), and an inset eyeball (enophthalmos). This syndrome is caused by disruption of the sympathetic nerves that run from the thoracic spine to the face. In the context of lung cancer, Horner's syndrome is most commonly caused by invasion of the sympathetic chain by a tumor, which in this case is likely a squamous cell carcinoma involving the apex of the left lung. The apex of the lung is a common site for SCC to arise, and its location allows it to invade the sympathetic chain and cause Horner's syndrome.

**Why Each Wrong Option is Incorrect**
**Option B:** Adenocarcinoma is the most common type of lung cancer, but it tends to arise in the peripheral airways, not the central airways, and is less likely to cause Horner's syndrome.
**Option C:** Small-cell carcinoma is a highly aggressive type of lung cancer that often presents with systemic symptoms such as paraneoplastic syndromes, but it is less likely to cause localized symptoms like Horner's syndrome.
**Option D:** Bronchioloalveolar carcinoma is a subtype of adenocarcinoma that tends to arise in the peripheral airways and is less likely to cause Horner's syndrome due to its location.

**Clinical Pearl / High-Yield Fact**
The location of a lung tumor can often be inferred from its symptoms. Tumors arising in the apical regions of the lung are more likely to cause Horner's syndrome due to their proximity to the sympathetic chain. This is a classic exam trap, and students should remember that the location of a lung tumor can often provide clues to its diagnosis.

**✓ Correct Answer: A. Squamous cell carcinoma involving the apex of his left lung**