**Core Concept**
Horner syndrome is a neurologic disorder characterized by a disruption in the sympathetic nerves supplying the eye. It leads to a triad of symptoms: ptosis (drooping eyelid), miosis (pupil constriction), and anhidrosis (lack of sweating). The clinical presentation of Horner syndrome is due to the impaired sympathetic innervation of the eye, which affects various structures such as the superior tarsal muscle, the dilator pupillae muscle, and the sweat glands of the face.

**Why the Correct Answer is Right**
Mydriasis, or pupil dilation, is not a characteristic feature of Horner syndrome. In fact, Horner syndrome is typically associated with miosis, which is the constriction of the pupil. This occurs due to the impaired sympathetic innervation of the dilator pupillae muscle, which normally causes pupil dilation. The loss of this sympathetic tone leads to a smaller pupil. The correct answer is mydriasis because it is the opposite of what is expected in Horner syndrome.

**Why Each Wrong Option is Incorrect**
**Option A:** Enophthalmos, or the posterior displacement of the globe of the eye, is a feature of Horner syndrome. It occurs due to the loss of sympathetic tone to the orbital smooth muscle, which normally helps to protrude the eye.

**Option C:** Anhidrosis, or the lack of sweating, is a feature of Horner syndrome. It occurs due to the interruption of the sympathetic nerves that innervate the sweat glands of the face.

**Option D:** Narrowed palpebral fissure, or the decreased width of the eyelid opening, is a feature of Horner syndrome. It occurs due to the ptosis (drooping eyelid) associated with the syndrome.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that Horner syndrome is often associated with other signs such as facial asymmetry, drooping eyelid, and a decreased ability to sweat on the affected side of the face. A thorough examination of these signs can help diagnose Horner syndrome.

**✓ Correct Answer: B. Mydriasis**