## **Core Concept**
The frequency of ultrasound used in ophthalmics depends on the specific application and the desired resolution and penetration. Ophthalmic ultrasound typically utilizes high-frequency sound waves to produce detailed images of the eye structures.

## **Why the Correct Answer is Right**
The correct answer, **10 MHz**, is commonly used in ophthalmic ultrasound because it provides a good balance between resolution and penetration. High-frequency ultrasound waves, such as those at 10 MHz, offer high-resolution images of the anterior segment and the posterior segment of the eye, including the retina and the optic nerve. This frequency range is optimal for detailed assessment of ocular structures.

## **Why Each Wrong Option is Incorrect**
* **Option A: 1 MHz** - This frequency is too low for detailed ophthalmic imaging. Lower frequencies are used for deeper tissue penetration but would not provide the necessary resolution for the small structures within the eye.
* **Option B: 5 MHz** - Although this frequency can be used for some ophthalmic applications, it is not as ideal as 10 MHz for detailed imaging of the eye's structures. It might be used for certain specific assessments but is not the most common choice.
* **Option D: 20 MHz** - While higher frequencies can provide even higher resolution, 20 MHz might offer less penetration and is not as commonly used for routine ophthalmic ultrasound examinations as 10 MHz.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the choice of ultrasound frequency in ophthalmics is a trade-off between **resolution** and **penetration**. Higher frequencies (like 10 MHz) are preferred for detailed imaging of the eye but may not penetrate as deeply as lower frequencies.

## **Correct Answer: C. 10 MHz**