## **Core Concept**
Ultrasonic devices utilize sound waves with frequencies higher than the upper audible limit of human hearing, which is typically considered to be above 20 kHz. These devices are used in various medical applications, including diagnostics (e.g., ultrasound imaging) and therapeutics (e.g., lithotripsy). The core principle behind ultrasonic devices is the generation and manipulation of high-frequency sound waves.

## **Why the Correct Answer is Right**
The correct answer, **C. 2-10 MHz**, corresponds to the typical frequency range used in medical ultrasound. This range allows for adequate resolution and penetration in soft tissues, making it suitable for imaging structures within the body. Frequencies within this range are used for B-mode ultrasound imaging, Doppler studies, and other ultrasound modalities.

## **Why Each Wrong Option is Incorrect**
- **Option A: 20 Hz - 20 kHz** - This range represents the audible spectrum for humans and is not considered ultrasonic. Ultrasonic frequencies are specifically those above 20 kHz.
- **Option B: 10 Hz - 20 kHz** - This option also falls within the human audible range and below the threshold for ultrasonic frequencies.
- **Option D: 100 MHz - 1000 MHz** - While higher frequency ultrasound exists and is used in some specialized applications (like ultrasound biomicroscopy), the range of 2-10 MHz is more standard and widely used for general medical ultrasound imaging.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the choice of ultrasound frequency affects both the resolution of the image and its penetration. Higher frequencies (e.g., 5-10 MHz) provide better resolution but less penetration, making them suitable for superficial structures. Lower frequencies (e.g., 2-3 MHz) offer deeper penetration but with less resolution, often used for deeper structures like the liver or gallbladder.

## **Correct Answer:** C. 2-10 MHz.