According to the ISNT rule, the nerve fiber layer is thickest in which quadrant?

• A. Inferior
• B. Superior
• C. Nasal
• D. Temporal

Answer: (A)

The question is about the ISNT rule for the peripapillary nerve fiber layer (RNFL) around the optic disc. This rule states that RNFL thickness follows a specific order: inferior is thickest, then superior, then nasal, and temporal is thinnest. The inferior quadrant ends up being the thickest because more nerve fibers converge and run through the inferior arc toward the optic nerve, giving that region greater bundle density.

In clinical practice, this pattern serves as a reference when evaluating RNFL thickness on OCT or red-free imaging. Deviations from the expected ISNT distribution, particularly thinning in the inferior quadrant, can be an early sign of glaucomatous damage. Therefore, the thickest RNFL quadrant is the inferior one.