## Core Concept
Horner's syndrome is a neurological disorder characterized by a classic triad of ptosis, miosis, and anhidrosis. It results from the disruption of the sympathetic nerves supplying the eye, often due to lesions in the hypothalamic-neuroectodermal pathways.

## Why the Correct Answer is Right
The correct answer is related to the typical features of Horner's syndrome. The syndrome is indeed characterized by ptosis (drooping eyelid), miosis (pupillary constriction), and anhidrosis (absence of sweating) on the affected side of the face.

## Why Each Wrong Option is Incorrect
- **Option A:** Ptosis is a correct association with Horner's syndrome. It occurs due to the interruption of sympathetic nerves supplying the superior tarsal muscle, which assists in elevating the eyelid.
- **Option B:** Miosis is also a characteristic feature. It results from the loss of sympathetic input to the dilator pupillae muscle, leading to unopposed parasympathetic action.
- **Option C:** Anhidrosis on the affected side of the face is another feature. This happens because of the disruption of sympathetic fibers that control sweat glands.

## Why Option D is Correct (Implicit)
Since the question asks for "all of the following except," and given that ptosis, miosis, and anhidrosis are characteristic features, any option not aligning with these features would be correct. Typically, **exophthalmos** (bulging of the eye) is not associated with Horner's syndrome; instead, enophthalmos (the sinking of the eye into the orbit) is.

## Clinical Pearl / High-Yield Fact
A key clinical pearl is that in Horner's syndrome, the ptosis is typically mild (drooping but not complete closure of the eye). Also, remember that anhidrosis might not be present in all cases, especially if the lesion is central (above the level of the superior cervical ganglion), because the sympathetic pathways for sweating diverge at a higher level.

## Correct Answer: D. exophthalmos