**Core Concept**
The echogenicity of a structure on ultrasonography refers to its ability to reflect ultrasound waves, resulting in a brighter appearance on the image. Different tissues have varying levels of echogenicity, which can help identify their nature.

**Why the Correct Answer is Right**
The liver is normally hyperechoic (brighter) on ultrasonography due to its high fat content. This characteristic echogenicity helps differentiate the liver from other abdominal organs. The liver's echogenicity is often used as a reference point to assess the echogenicity of surrounding structures.

**Why Each Wrong Option is Incorrect**
**Option A:** Blood is anechoic (black) because it does not reflect ultrasound waves well, making it difficult to visualize on ultrasound images.
**Option B:** Gas-filled structures, such as the bowel, are hyperechoic due to the reflection of ultrasound waves from the air-gas interface.
**Option C:** The spleen is hypoechoic (darker) compared to the liver, due to its higher water content and lower fat content.

**Clinical Pearl / High-Yield Fact**
When interpreting abdominal ultrasound images, it's essential to remember that the liver is usually hyperechoic, while the spleen is hypoechoic. This helps differentiate these two organs and can aid in identifying pathology.

**Correct Answer: D. Blood is anechoic.**