## **Core Concept**
The core concept being tested here is the application of ultrasound technology in obstetrics, specifically focusing on the frequency range used for diagnostic purposes. Ultrasound frequencies are crucial in determining the resolution and penetration of the ultrasound waves in tissues.

## **Why the Correct Answer is Right**
The correct answer, **3-5 MHz**, is suitable for obstetric imaging because it provides a good balance between resolution and penetration. This frequency range is commonly used for transabdominal ultrasound scans during pregnancy. It allows for clear visualization of the fetus, placenta, and other maternal structures. Frequencies in this range are not too high, which would limit penetration, nor too low, which would compromise resolution.

## **Why Each Wrong Option is Incorrect**
- **Option A: 1-2 MHz** - This frequency range is on the lower side for detailed obstetric imaging. It's often used for deeper tissue penetration but would provide lower resolution images of the fetus and maternal structures.
- **Option B: 5-7 MHz** - While this range offers higher resolution and is used in obstetrics, particularly for transvaginal ultrasound or detailed fetal anatomy scans, it's not the broad range typically cited for general diagnostic purposes in obstetrics.
- **Option D: 10-15 MHz** - This high-frequency range provides very high resolution but has limited penetration, making it less suitable for general obstetric imaging, especially in later pregnancy stages.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the choice of ultrasound frequency depends on the gestational age and the specific information needed. For example, **transvaginal ultrasound** often uses higher frequencies (5-7 MHz) for better resolution in early pregnancy, while **transabdominal ultrasound** uses lower frequencies (3-5 MHz) for deeper penetration.

## **Correct Answer:** . **3-5 MHz**