**Core Concept**
The Marcus Gunn sign, also known as a relative afferent pupillary defect (RAPD), is a clinical indicator of an afferent pupillary defect, typically caused by a disorder affecting the optic nerve or retina. This sign is observed during the swinging-flashlight test, where a difference in pupillary reaction is noted between the two eyes.

**Why the Correct Answer is Right**
The Marcus Gunn sign is a result of the impaired afferent input from the affected eye, leading to a reduced pupillary constriction in response to light. This occurs due to the disruption of the afferent pathway from the retina to the Edinger-Westphal nucleus in the midbrain. The affected eye cannot transmit the light signal effectively, causing an abnormal pupillary response.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not relevant to the Marcus Gunn sign, as it refers to a different clinical phenomenon.
* **Option B:** This option might seem plausible but is incorrect, as the Marcus Gunn sign is not a result of a central nervous system lesion.
* **Option C:** This option is incorrect, as the Marcus Gunn sign is not a manifestation of a specific disease, but rather a clinical indicator of an underlying afferent pupillary defect.

**Clinical Pearl / High-Yield Fact**
The Marcus Gunn sign is an essential clinical finding in the evaluation of patients with visual field defects, optic neuropathies, or other conditions affecting the afferent visual pathway. It is crucial to recognize this sign during the swinging-flashlight test to identify potential underlying conditions.

**Correct Answer:** C