**Core Concept**
A Marcus Gunn pupil is a type of relative afferent pupillary defect (RAPD) characterized by a dilated pupil in response to a light shone in the affected eye, which then constricts when the light is moved to the normal eye. This phenomenon occurs due to a difference in afferent pupillary pathways between the two eyes.

**Why the Correct Answer is Right**
The Marcus Gunn pupil is typically seen in conditions affecting the optic nerve, such as optic neuritis, or in cases of severe retinal disease. When light is shone in the affected eye, the pupil dilates due to the impaired afferent input. However, when the light is moved to the normal eye, the intact afferent pathway causes the pupil to constrict.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is likely to be incorrect, but without the actual options, it's hard to provide a specific explanation. However, based on the context, we can infer that options A, C, and D are conditions that may not typically present with a Marcus Gunn pupil.
* **Option B:** This option is likely to be the correct answer, as it is a condition that is not typically associated with a Marcus Gunn pupil. However, without knowing the specific options, it's hard to provide a detailed explanation.

**Clinical Pearl / High-Yield Fact**
One key point to remember is that a Marcus Gunn pupil is often seen in conditions that affect the afferent pupillary pathways, such as optic neuritis or severe retinal disease. It's essential to differentiate this from other causes of pupillary abnormalities, such as third nerve palsy.

**Correct Answer:** B.