**Question:** In open Angle Glaucoma which investigation is least useful –

A. Retinal nerve fiber layer (RNFL) thickness measurement
B. Intraocular pressure (IOP) measurement
C. Fundus examination
D. Visual field testing

**Core Concept:**
Open angle glaucoma is a chronic eye disease characterized by progressive damage to the optic nerve due to increased intraocular pressure (IOP), leading to optic nerve head atrophy and visual field defects. This condition primarily involves the anterior chamber angle where there is no obstruction to the outflow of aqueous humor, hence the term "open angle."

**Why the Correct Answer is Right:**
In open angle glaucoma, the main risk factor is the elevated IOP, which directly damages the optic nerve. Therefore, the investigation least useful in this context would be one that does not directly address IOP or its consequences.

**Why Each Wrong Option is Incorrect:**
A. Retinal nerve fiber layer (RNFL) thickness measurement: Assesses the structural damage caused by glaucoma, but it does not directly address the primary risk factor (IOP) and is less specific to open angle glaucoma.
B. Visual field testing: Evaluates the patient's visual field defects, which may be affected by glaucoma but does not directly address the primary risk factor (IOP).
C. Fundus examination: Assesses the optic nerve head appearance and retinal blood vessels, which can indicate glaucoma progression but does not directly address the primary risk factor (IOP).
D. Visual field testing: Evaluates the patient's visual field defects, which may be affected by glaucoma but does not directly address the primary risk factor (IOP).

**Clinical Pearl:**
The least useful investigation in open angle glaucoma is visual field testing, as it assesses visual field defects secondary to glaucoma but does not directly address the primary risk factor (IOP). For a comprehensive evaluation of glaucoma, a combination of IOP measurement, optic nerve head examination, RNFL thickness measurement, and visual field testing should be performed. Regular monitoring of these investigations is essential for early detection and management of glaucoma to prevent blindness and preserve visual function.