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

A. Intraocular Pressure (IOP) measurement
B. Fundoscopy
C. Pupillary Dilator Spread (PDS)
D. Visual Field Testing

**Core Concept:**
Open Angle Glaucoma is a type of glaucoma characterized by an elevated intraocular pressure (IOP) leading to optic nerve damage and visual field loss. This condition is typically diagnosed through a combination of IOP measurement, fundoscopy, PDS, and visual field testing.

**Why the Correct Answer is Right:**
The least useful investigation in open angle glaucoma is Pupillary Dilator Spread (PDS), as its main function is to assess the constriction and dilation of the pupils in response to light, which is not directly related to the diagnosis or monitoring of glaucoma progression.

**Why Each Wrong Option is Incorrect:**

A. Intraocular Pressure (IOP) measurement: Ongoing IOP monitoring is crucial in managing open angle glaucoma, as it provides vital information about the disease progression and response to treatment.

B. Fundoscopy: This examination helps visualize the optic nerve head and retinal vessels, which are important for assessing optic nerve health but is not directly related to the diagnosis of glaucoma or IOP assessment.

C. Pupillary Dilator Spread (PDS): As mentioned before, PDS is not directly related to the diagnosis or monitoring of glaucoma progression.

D. Visual Field Testing: Visual field testing is essential in diagnosing and monitoring glaucoma, as it assesses the visual field loss due to optic nerve damage, which is a hallmark of glaucoma progression.

**Clinical Pearl:**

In open angle glaucoma, a thorough eye examination should include IOP measurement, fundoscopy, and visual field testing. While PDS is helpful in assessing pupillary function, it is not a primary tool in diagnosing or monitoring glaucoma progression.