## **Core Concept**
Open-angle glaucoma is a condition characterized by a slow clogging of the drainage canals, resulting in increased eye pressure. It is a leading cause of irreversible blindness worldwide. The diagnosis and management of open-angle glaucoma involve various investigations to assess the optic nerve, visual field, and intraocular pressure.

## **Why the Correct Answer is Right**
The correct answer, Gonioscopy, is least useful in open-angle glaucoma because it is primarily used to examine the anterior chamber angle for abnormalities such as angle closure, and to identify the type of glaucoma. In open-angle glaucoma, the angle appears wide and open on gonioscopy, and the test does not provide significant additional information for diagnosis or management.

## **Why Each Wrong Option is Incorrect**
* **Option A:** Visual field testing is crucial in open-angle glaucoma as it helps detect the characteristic arcuate scotomas and nasal steps, which are indicative of glaucomatous damage to the optic nerve.
* **Option B:** Optic disc photography is useful for assessing the optic disc for signs of glaucomatous damage such as cupping, which is a hallmark of glaucoma.
* **Option C:** Tonometry, specifically applanation tonometry, is essential for measuring intraocular pressure (IOP), a key diagnostic criterion for glaucoma.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that in open-angle glaucoma, the focus is on assessing the optic nerve head for signs of damage (like cupping) and monitoring intraocular pressure (IOP) levels. Gonioscopy, while useful in angle closure glaucoma, does not significantly contribute to the diagnosis or management of open-angle glaucoma.

## **Correct Answer:** . Gonioscopy