Question

According to the ISNT rule, what is the thinnest part of the neuro-retinal rim?

• A. Inferior
• B. Superficial
• C. Temporal
• D. Medial

Hint:

ISNT goes thickest to thinnest; the last letter is the answer.

Answer 1

The Correct Option is C

Step 1: Recall the ISNT rule. The ISNT rule describes the normal pattern of neuro-retinal rim thickness around the optic disc, going from thickest to thinnest as Inferior, Superior, Nasal, Temporal.

Step 2: Apply it to the question. Since the order ends with Temporal, the neuro-retinal rim is normally thickest inferiorly and thinnest temporally.

Step 3: Clinical relevance. In glaucoma this pattern is lost, with vertical thinning and atrophy along the inferior and superior rims, so the ISNT rule is broken.

Step 4: Why the distractors are wrong. Inferior is the thickest, not the thinnest. Superficial and medial are not part of the ISNT sequence, so they are incorrect.