**Question:** In "obsetrics ultra sound" commonly employed range of frequency is:-

A. 2-6 MHz
B. 6-10 MHz
C. 5-12 MHz
D. 20-100 MHz

**Core Concept:**
In obstetric ultrasound, the choice of frequency depends on the desired depth of penetration and resolution of the image. Higher frequencies provide better resolution but shallower penetration, while lower frequencies offer deeper penetration but at the cost of image resolution.

**Why the Correct Answer is Right:**
The correct answer, C. 5-12 MHz, is chosen for obstetric ultrasound because it strikes a balance between penetration depth and image resolution. This frequency range allows for adequate penetration to visualize the fetal anatomy, while maintaining sufficient image resolution to differentiate between structures.

**Why Each Wrong Option is Incorrect:**
A. 2-6 MHz: Lower frequencies may not penetrate the uterine wall sufficiently for accurate fetal evaluation.
B. 6-10 MHz: Higher frequencies may not provide adequate penetration, resulting in poor visualization of deeper structures.
D. 20-100 MHz: Extremely high frequencies are not suitable for obstetric ultrasound due to limited penetration depth and excessive tissue attenuation.

**Clinical Pearl:**
The choice of ultrasound frequency is crucial for optimal image quality in obstetric ultrasound. Understanding the relationship between frequency and image resolution vs penetration depth is essential for accurate diagnosis and assessment of the fetus.

**Correct Answer:** C. 5-12 MHz