**Core Concept**
Open-angle glaucoma is a progressive optic neuropathy characterized by increased intraocular pressure (IOP) due to impaired aqueous humor outflow. The primary goal of investigation is to diagnose and manage glaucoma to prevent vision loss.

**Why the Correct Answer is Right**
The least commonly performed investigation in open-angle glaucoma is the measurement of corneal thickness using pachymetry. This is because most patients with open-angle glaucoma have normal corneal thickness, and corneal thickness does not directly correlate with the risk of glaucomatous damage. However, pachymetry is essential in patients with suspected glaucoma who have a history of ocular trauma or surgery, as it helps in assessing the risk of glaucoma.

**Why Each Wrong Option is Incorrect**
**Option A:** Tonometry is a crucial investigation in open-angle glaucoma, as it measures IOP, which is a key risk factor for glaucomatous damage. Tonometry is performed using techniques such as applanation tonometry or rebound tonometry.

**Option B:** Perimetry, including visual field testing, is an essential investigation in open-angle glaucoma, as it helps in detecting early signs of glaucomatous damage, such as visual field defects.

**Option C:** Optical coherence tomography (OCT) is a non-invasive imaging technique that provides detailed images of the retinal nerve fiber layer and optic disc, helping in diagnosing and monitoring glaucoma.

**Clinical Pearl / High-Yield Fact**
It's essential to note that the diagnosis of open-angle glaucoma is primarily based on the combination of elevated IOP, optic disc damage, and visual field defects. A single investigation may not be sufficient to diagnose glaucoma, and a comprehensive approach is necessary.

**Correct Answer: B. Perimetry, including visual field testing, is an essential investigation in open-angle glaucoma, as it helps in detecting early signs of glaucomatous damage, such as visual field defects.