**Core Concept**
In ophthalmology, tonometry involves measuring the intraocular pressure (IOP) of the eye. A topical anesthetic is used to numb the cornea, allowing for a more accurate measurement. Xylocaine (lidocaine) is a commonly used local anesthetic in this context.

**Why the Correct Answer is Right**
A 4% xylocaine solution is typically used as a topical anesthetic for tonometry. This concentration is sufficient to anesthetize the cornea without causing significant systemic absorption or toxicity. The anesthetic effect of xylocaine is mediated by its ability to block sodium channels in nerve fibers, preventing the transmission of pain signals. In the context of tonometry, the goal is to achieve a rapid and effective anesthetic effect to minimize discomfort and ensure accurate IOP measurement.

**Why Each Wrong Option is Incorrect**
**Option B:** 3% xylocaine is not typically used for tonometry, as it may not provide sufficient anesthesia for the procedure.
**Option C:** 2% xylocaine is often used for minor surgical procedures, but it may not be potent enough for tonometry.
**Option D:** 1% xylocaine is a common concentration used for surface anesthesia, but it is not strong enough for tonometry.

**Clinical Pearl / High-Yield Fact**
When using xylocaine for tonometry, it's essential to apply a single drop to the center of the cornea, allowing it to spread evenly before measuring IOP. This helps ensure accurate results and minimizes the risk of toxicity.

**✓ Correct Answer: A. 4% xylocaine**