**Core Concept**
Corneal thickness is a critical parameter in ophthalmology, particularly in the diagnosis and management of conditions like glaucoma, where thinning of the cornea can lead to increased intraocular pressure. Accurate measurement of corneal thickness is essential for assessing the risk of glaucoma progression and guiding treatment decisions.

**Why the Correct Answer is Right**
Pachymetry is a non-invasive technique used to measure the thickness of the cornea. It involves the use of a specialized instrument, such as an ultrasound pachymeter or an optical pachymeter, which emits a high-frequency sound wave or light beam that passes through the cornea. The time taken for the sound wave or light beam to travel through the cornea is directly proportional to its thickness. This measurement is essential for assessing the corneal thickness and for diagnosing conditions like glaucoma, where the cornea is thinner than normal.

**Why Each Wrong Option is Incorrect**
**Option B:** A keratometer is an instrument used to measure the curvature of the cornea, not its thickness. While it is used in ophthalmology to assess the cornea, it is not suitable for measuring corneal thickness.
**Option C:** A CT scan is a diagnostic imaging modality that provides detailed cross-sectional images of the body, including the eyes. However, it is not typically used to measure corneal thickness, as it is not sensitive enough to detect small changes in corneal thickness.
**Option D:** Aesthisiometry is a term not commonly used in ophthalmology, and it does not relate to the measurement of corneal thickness. The correct term is aesthesiometry, which refers to the measurement of corneal sensitivity.

**Clinical Pearl / High-Yield Fact**
It is essential to measure corneal thickness in patients with glaucoma, as thinning of the cornea can lead to increased intraocular pressure and worsen the disease. Pachymetry is a simple, non-invasive technique that provides accurate measurements of corneal thickness, making it an essential tool in the diagnosis and management of glaucoma.

**✓ Correct Answer: A. Pachymetry**