**Question:** You have been referred a middle-aged patient to rule out open angle glaucoma. Which of the following findings will help in the diagnosis –

A. Increased intraocular pressure
B. Notched or irregular corneal margin
C. Anterior chamber angle narrowing
D. Thinning of the peripheral retina

**Core Concept:**
Open angle glaucoma is a type of glaucoma characterized by an increased intraocular pressure (IOP) leading to optic nerve damage. The optic nerve is primarily affected due to elevated pressure within the eye, which prevents proper blood flow to the optic nerve head.

**Why the Correct Answer is Right:**
A. Increased intraocular pressure (IOP) is a crucial finding in diagnosing open angle glaucoma. Elevated IOP directly damages the optic nerve, leading to the characteristic optic nerve damage seen in glaucoma.

**Why Each Wrong Option is Incorrect:**
B. Notched or irregular corneal margin is a feature of keratoconus, a corneal ectasia disorder, not open angle glaucoma. Keratoconus is characterized by a thin, cone-shaped cornea resulting in a notched or irregular corneal margin.
C. Anterior chamber angle narrowing is a feature of closed angle glaucoma, not open angle glaucoma. In closed angle glaucoma, the angle between the cornea and iris is narrowed, leading to obstruction of the aqueous humor outflow and increased IOP.
D. Thinning of the peripheral retina is observed in retinal disorders like optic neuritis or macular degeneration, not open angle glaucoma. Retinal thinning is not a characteristic feature of glaucoma.

**Clinical Pearl:**
Understanding the specific features of various glaucoma types (e.g., open angle, closed angle, and normal tension) is crucial in diagnosing and managing glaucoma effectively.

In conclusion, the correct answer is A (Increased intraocular pressure) as it is the primary feature and causative factor in open angle glaucoma. The other options represent different conditions with distinct clinical features.