**Core Concept**
Horner's syndrome is a neurologic disorder resulting from disruption of the sympathetic nerves supplying the eye, leading to a characteristic set of clinical features. The affected individual typically presents with a triad of ptosis, miosis, and enophthalmos.

**Why the Correct Answer is Right**
Cycloplegia, which is the paralysis of the ciliary muscles, is not a feature of Horner's syndrome. This is because Horner's syndrome primarily affects the sympathetic nerves, which control the dilation of the pupil and the elevation of the eyelid, but not the parasympathetic nerves, which control the constriction of the pupil and the contraction of the ciliary muscles. The absence of cycloplegia in Horner's syndrome is due to the selective involvement of the sympathetic pathways.

**Why Each Wrong Option is Incorrect**
**Option A:** Miosis is a characteristic feature of Horner's syndrome, resulting from the disruption of the sympathetic nerves that normally dilate the pupil.
**Option B:** Enophthalmos, or the posterior displacement of the globe, is also a feature of Horner's syndrome, resulting from the loss of sympathetic tone to the orbital smooth muscles.
**Option C:** Ptosis, or drooping of the eyelid, is another classic feature of Horner's syndrome, resulting from the weakness of the superior tarsal muscle due to sympathetic denervation.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that Horner's syndrome can be caused by a variety of conditions, including carotid artery dissection, pancreatic cancer, and brainstem lesions, making a thorough evaluation and history taking critical for diagnosis.

**✓ Correct Answer: D. Cycloplegia**