**Core Concept**
Retinoscopy is a technique used to determine the refractive error of the eye, particularly in infants and young children who are unable to communicate their vision problems. This method involves shining a light into the eye and observing the reflection to estimate the refractive error and prescribe corrective lenses.

**Why the Correct Answer is Right**
Retinoscopy works by using a retinoscope, which projects a beam of light into the eye. The reflection from the retina is observed, and the movement of the reflection in response to changes in the light source is used to estimate the refractive error. The power of the light source is adjusted to match the refractive error of the eye, resulting in a static reflex. This technique is particularly useful in pediatric ophthalmology, as it allows for the accurate measurement of refractive error in children who are unable to communicate their vision problems.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because retinoscopy is not primarily used for diagnosing eye diseases, although it can provide information about the health of the retina.
* **Option B:** This option is incorrect because retinoscopy does not involve the use of a direct ophthalmoscope, which is a different type of instrument used to examine the retina.
* **Option C:** This option is incorrect because retinoscopy is not used for measuring intraocular pressure, which is the primary function of a tonometer.

**Clinical Pearl / High-Yield Fact**
Retinoscopy is a valuable tool in pediatric ophthalmology, allowing for the accurate measurement of refractive error in children who are unable to communicate their vision problems. It is essential to note that retinoscopy should only be performed by trained ophthalmologists or optometrists.

**Correct Answer: C. Measuring intraocular pressure.**