Okay, let's tackle this question about the frequency of sound waves used in transabdominal ultrasonography. First, I need to recall the basics of ultrasound physics. Ultrasound frequencies are categorized based on their application. Higher frequencies provide better resolution but less penetration, while lower frequencies penetrate deeper but have lower resolution.

Transabdominal ultrasound is used for imaging abdominal organs like the liver, kidneys, and gallbladder. Since the abdominal wall is relatively thick, the ultrasound needs to penetrate deeper. Therefore, lower frequencies are preferred here. Common frequencies for abdominal ultrasounds are around 2-5 MHz. Let me check the options again. The correct answer should be in that range.

Now, the incorrect options might be higher frequencies like 7-15 MHz, which are used for superficial structures such as the thyroid or musculoskeletal areas. Another possible wrong answer could be even lower, like below 2 MHz, but those are used in cardiac or obstetric ultrasounds for deeper penetration. Also, frequencies above 15 MHz are too high for abdominal use because they don't penetrate deeply enough.

The key point here is understanding the inverse relationship between frequency and penetration depth. Students should remember that abdominal imaging requires lower frequencies for better tissue penetration, while superficial structures require higher frequencies. This is a classic exam topic where confusion might arise between different ultrasound applications.

**Core Concept**
The frequency of ultrasound waves determines their **penetration depth** and **resolution**. Lower frequencies (2–5 MHz) are used for abdominal imaging to balance penetration and resolution, while higher frequencies (7–15 MHz) are used for superficial structures.

**Why the Correct Answer is Right**
Transabdominal ultrasonography requires **2–5 MHz** sound waves to penetrate abdominal tissues effectively. Lower frequencies reduce attenuation in soft tissues, allowing visualization of deeper structures like the liver or kidneys. Higher frequencies would lack sufficient penetration, while lower than 2 MHz would compromise resolution.

**Why Each Wrong Option is Incorrect**
**Option A:** 7–15 MHz (Incorrect) – These high frequencies are used for superficial imaging (e.g., thyroid, musculoskeletal), not abdominal structures.
**Option B:** 1–2 MHz (Incorrect) – Frequencies 15 MHz (Incorrect) – Extremely high frequencies are used in superficial, high-resolution imaging (e.g., dermatology) and cannot penetrate abdominal tissues.

**Clinical Pearl / High-Yield Fact**
Remember: **Lower frequency = deeper penetration** (abdomen/kidneys), **higher frequency = better resolution** (thyroid/testes). Abdominal ultrasound uses **2–5 MHz**, while musculoskeletal uses **7–15 MHz**.

**Correct Answer: C. 2–5 MHz**

✓ Correct Answer: B. 3.5-5.0 MHz