Okay, let's tackle this question. The patient has a reddish eye, watering, and a shallow anterior chamber. The key here is to figure out the underlying condition causing these symptoms. Red eye with a shallow chamber makes me think of angle-closure glaucoma. That's a common scenario where the anterior chamber becomes shallow due to the iris blocking the trabecular meshwork, leading to increased intraocular pressure.

The next step in investigation would be to confirm if there's an acute angle-closure. The gold standard for this is gonioscopy, which allows direct visualization of the irido-corneal angle. If the angle is closed, that confirms the diagnosis. Alternatively, if the options include tonometry to measure intraocular pressure, that's also important. But since the question mentions a shallow anterior chamber, which is a structural finding, gonioscopy is more specific here.

Wait, but sometimes the options might include other tests like slit lamp examination. However, a slit lamp can assess the chamber depth, but gonioscopy is needed to determine if the angle is closed. Also, considering that the patient has watering and red eye, which can be due to a painful eye in acute glaucoma, the combination of symptoms and shallow chamber points more towards angle closure. So the next investigation should be gonioscopy to confirm the diagnosis. Let me check the options again. The correct answer is likely gonioscopy.

**Core Concept**
The clinical scenario describes **acute angle-closure glaucoma**, characterized by a **red eye**, **pain**, **watering**, and a **shallow anterior chamber**. The key pathophysiology involves abrupt closure of the irido-corneal angle, obstructing aqueous outflow and causing rapid intraocular pressure (IOP) elevation.

**Why the Correct Answer is Right**
**Gonioscopy** is the definitive investigation to visualize the irido-corneal angle and confirm angle closure. It distinguishes between open-angle and closed-angle glaucoma. In acute angle-closure, the angle appears closed due to apposition of the iris to the trabecular meshwork, preventing aqueous drainage. This test is critical for guiding urgent treatment (e.g., laser iridotomy).

**Why Each Wrong Option is Incorrect**
**Option A:** *Slit lamp examination* can assess chamber depth but cannot confirm angle closure without gonioscopic optics.
**Option B:** *Intraocular pressure measurement* may show elevated IOP but does not confirm the mechanism (e.g., angle closure vs. secondary causes).
**Option C:** *Fundoscopy* might reveal optic nerve head changes but is not diagnostic in acute presentations.

**Clinical Pearl / High-Yield Fact**
**"Red eye with shallow anterior chamber = acute angle-closure glaucoma"** is a classic exam trap. Remember that **gonioscopy** is the gold standard to confirm angle status, not just IOP or chamber depth. Delaying this test can lead to permanent vision loss.

**Correct Answer: C. Gonioscopy**

✓ Correct Answer: A. Tonometry