**Core Concept:** Primary open-angle glaucoma is a chronic eye disease characterized by progressive optic neuropathy, leading to vision loss. It is caused by the accumulation of aqueous humor, a clear fluid produced by the ciliary body, in the anterior chamber of the eye due to an imbalance between production and drainage. The characteristic visual field defect in glaucoma is a localized constriction of the visual field, often leading to a "stepped wedge" pattern, where the visual field loss is more severe in the superior and inferior fields compared to the temporal and nasal fields.

**Why the Correct Answer is Right:** The correct answer is B, the "stepped wedge" defect. In primary open-angle glaucoma, the characteristic visual field defect is a localized constriction of the visual field, which is more severe in the superior and inferior visual fields compared to the temporal and nasal fields. This "stepped wedge" defect is due to the uneven distribution of retinal ganglion cell axons in the optic nerve head. The axons from the nasal and temporal retina are more numerous than the axons from the superior and inferior retina, leading to the observed pattern of visual field loss.

**Why Each Wrong Option is Incorrect:**

A. This is incorrect because glaucoma primarily affects the retinal ganglion cells, causing a more uniform pattern of visual field loss.
C. This option is incorrect because the "cone" defect refers to a central scotoma, which is not specific to glaucoma.
D. This is incorrect as the "crescent" defect refers to a central scotoma, which is not characteristic of glaucoma. The "stepped wedge" defect is unique to glaucoma and helps to diagnose the condition early.

**Clinical Pearl:** The stepped wedge visual field defect in glaucoma is a critical clue for early diagnosis and timely management of this potentially blinding condition. Early intervention is crucial for slowing down the progression of visual field loss and preserving vision.