**Core Concept**
Hutchinson's pupil is a clinical sign associated with a specific type of eye abnormality that can be observed in patients with certain neurological conditions. It is characterized by a pinpoint pupils that react sluggishly or not at all to light. This phenomenon is due to the involvement of the oculomotor nerve, which is responsible for controlling the movement and size of the pupils.

**Why the Correct Answer is Right**
Hutchinson's pupil is typically seen in patients with a third nerve palsy, which is often caused by an aneurysm of the internal carotid artery or an aneurysm of the posterior communicating artery. The aneurysm compresses the oculomotor nerve, leading to a dilated pupil that is fixed to light on the affected side. However, in some cases, the pupil may be constricted due to the presence of a relative afferent pupillary defect (RAPD), which is a condition where the pupil does not constrict properly in response to light due to damage to the afferent (sensory) pathway of the optic nerve.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because Hutchinson's pupil is not typically associated with a tonic pupil, which is a type of pupil abnormality that is characterized by a slow, gradual constriction of the pupil in response to light.
* **Option B:** This option is incorrect because Hutchinson's pupil is not typically seen in patients with a Horner's syndrome, which is a condition that affects the sympathetic nerves that supply the eye and is characterized by a constricted pupil, drooping eyelid, and facial weakness.
* **Option C:** This option is incorrect because Hutchinson's pupil is not typically associated with a Marcus Gunn pupil, which is a type of pupil abnormality that is characterized by a constricted pupil on the affected side when the patient looks towards the affected side.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that Hutchinson's pupil is a red flag for an aneurysm of the internal carotid artery or the posterior communicating artery, and patients with this clinical sign should be promptly evaluated for these conditions.

**Correct Answer: C.**