Okay, let's tackle this question. The user is asking about the earliest visual field change in open-angle glaucoma. I need to recall the typical progression of glaucoma. I remember that glaucoma affects the optic nerve, leading to visual field defects. The earliest changes usually occur in the peripheral vision, specifically in the nasal side because the retinal ganglion cells in that area are more susceptible.

Wait, but which specific part? Is it the nasal step, or the paracentral scotoma? I think the earliest is a nasal step in the visual field. That's when the patient has a loss of vision along the nasal side when looking straight ahead. Then it progresses to paracentral scotomas, followed by arcuate scotomas, and eventually tunnel vision.

So the correct answer here would be the nasal step. Let me check the options again. The user didn't list the options, but the correct answer is option C. So the explanation should focus on why nasal step is the earliest. The other options might be things like central scotoma, arcuate scotoma, or temporal hemianopia. Those come later. Central scotoma would be in the macula, which is not typically affected early. Arcuate scotomas are classic but later signs. Temporal hemianopia is more associated with optic nerve issues, not glaucoma.

The core concept here is the progression of visual field loss in open-angle glaucoma. The key is that the damage starts in the nasal periphery because of the distribution of the retinal ganglion cell axons. The nasal retina corresponds to the temporal visual field, and since the optic nerve is organized with nasal fibers in the temporal quadrant, damage there leads to nasal steps. That's why the earliest defect is a nasal step.

Now, for the incorrect options. If an option says "central scotoma," that's wrong because central vision is preserved early. "Arcuate scotoma" is a later stage. "Temporal hemianopia" is more from optic nerve lesions. "Enlarged blind spot" might be a distractor but isn't the earliest change.

The clinical pearl here is to remember that nasal steps are the first sign. Also, knowing that the visual field defects progress from the periphery inward is key for exams. So the correct answer is C, nasal step.

**Core Concept**
Open-angle glaucoma causes progressive optic neuropathy due to retinal ganglion cell death. Early visual field loss occurs in the nasal periphery because of the unique anatomical arrangement of optic nerve fibers, where nasal retinal fibers (projecting to the temporal visual field) are most vulnerable to ischemic damage.

**Why the Correct Answer is Right**
The earliest visual field defect in open-angle glaucoma is a **nasal step** (a localized loss of vision along the nasal field). This occurs because the nasal retina’s axons (which innervate the temporal visual field) travel centrally through the optic nerve and are compressed first by elevated intraocular pressure. Over time, this progresses to paracentral scotomas and arcuate scotomas. The nasal step is detectable via automated perimetry and precedes more advanced field loss.

**Why Each Wrong Option is Incorrect**
**Option A:** Central scotoma – Incorrect. Central vision (macula) is preserved until late

✓ Correct Answer: A. Paracentral scotoma