**Core Concept**
Apical lung tumors can compress or invade the sympathetic trunk, leading to Horner's syndrome, a condition characterized by ptosis, miosis, and anhidrosis (loss of sweating). This occurs due to disruption of the sympathetic nerves supplying the eye and face.

**Why the Correct Answer is Right**
The apical lung tumor compresses or invades the sympathetic trunk, specifically the superior cervical ganglion, which is responsible for transmitting sympathetic signals to the face. This leads to a disruption in the sympathetic fibers that control sweating, resulting in anhidrosis on the same side of the face. The affected fibers are pre-ganglionic, as they originate from the spinal cord and synapse in the superior cervical ganglion before reaching the face.

**Why Each Wrong Option is Incorrect**
**Option A:** Central Horner syndrome is typically associated with a brainstem lesion, whereas apical lung tumors cause peripheral Horner syndrome due to compression or invasion of the sympathetic trunk.

**Option B:** Postganglionic fibers are involved in Horner syndrome caused by apical lung tumors, but the primary effect is on the pre-ganglionic fibers that synapse in the superior cervical ganglion.

**Option C:** First-degree neuron injury refers to damage to the peripheral nerve, which is not the primary mechanism of Horner syndrome caused by apical lung tumors. The tumors affect the sympathetic trunk, which is a second-order neuron.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that apical lung tumors can cause Horner's syndrome due to compression or invasion of the sympathetic trunk, and this occurs because of pre-ganglionic fiber involvement. This is a classic exam trap, and students should be able to distinguish between central and peripheral causes of Horner syndrome.

**Correct Answer Line**
✓ Correct Answer: D. Loss of sweating on same side of face.