## **Core Concept**
Tonometry is a method used in ophthalmology to measure the intraocular pressure (IOP) of the eye. Different types of tonometers are available, each with its own mechanism and application, such as applanation tonometry and indentation tonometry. The question focuses on measuring IOP on an irregular corneal surface.

## **Why the Correct Answer is Right**
The correct answer, **D.**, refers to a specific type of tonometer suitable for irregular corneal surfaces. The **Tono-Pen** is a type of applanation tonometer that can be used on irregular surfaces and is less affected by the variations in corneal thickness and curvature compared to other methods. It provides a reliable measurement of IOP in such cases.

## **Why Each Wrong Option is Incorrect**
- **Option A:** The **Goldmann applanation tonometer** is a precise method for measuring IOP but requires a regular, clear corneal surface for accurate measurements. It might not be ideal for irregular surfaces.
- **Option B:** The **Schiøtz indentation tonometer** works by indenting the cornea and measuring the indentation. While it can be used for irregular corneas, it's less commonly used today due to its potential for causing discomfort and its lower accuracy compared to modern applanation tonometers.
- **Option C:** This could potentially refer to other types of tonometers, but without specificity, it's hard to directly refute. However, given the context, **D.** is more appropriate for irregular surfaces.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that for patients with irregular corneal surfaces, such as those with keratoconus or post-surgical irregularities, the **Tono-Pen** (or other handheld tonometers) can be particularly useful. These devices are portable and can provide IOP measurements where traditional tonometers might be less accurate.

## **Correct Answer: D. Tono-Pen**