**Core Concept**
Homer syndrome, also known as Horner's syndrome, is a neurologic disorder characterized by a disruption in the sympathetic nerves supplying the eye. This results in a triad of symptoms: ptosis (drooping eyelid), miosis (constricted pupil), and anhidrosis (absence of sweating) on the affected side of the face.

**Why the Correct Answer is Right**
The correct answer is related to the pathophysiology of Horner's syndrome, which involves a disruption in the sympathetic pathway that controls the eye. This pathway is mediated by the neurotransmitter norepinephrine, which is released by the sympathetic nerves. The disruption in this pathway leads to a decrease in norepinephrine release, resulting in the characteristic symptoms of Horner's syndrome.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because anhidrosis (absence of sweating) is actually a feature of Horner's syndrome, not its absence.

**Option B:** This option is incorrect because miosis (constricted pupil) is a hallmark symptom of Horner's syndrome, not its absence.

**Option C:** This option is incorrect because ptosis (drooping eyelid) is a characteristic feature of Horner's syndrome, not its absence.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that Horner's syndrome can be caused by a variety of conditions, including strokes, tumors, and trauma to the sympathetic nerves. A classic way to remember the symptoms of Horner's syndrome is by using the mnemonic "PTA" (ptosis, anhidrosis, and miosis).

**Correct Answer:** D