**Core Concept**
Water-soluble contrast media are a class of iodinated compounds used in various medical imaging modalities, including radiography, computed tomography (CT), and fluoroscopy. These agents are designed to be rapidly excreted by the kidneys, allowing for rapid imaging and minimizing the risk of nephrotoxicity.

**Why the Correct Answer is Right**
The correct answer is a type of water-soluble contrast agent that is commonly used in medical imaging. It is made up of a mixture of iodine and a non-ionic, water-soluble molecule, which allows it to be rapidly excreted by the kidneys and distributed throughout the body. The iodine component provides high X-ray attenuation, making it an effective contrast agent for various imaging modalities.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it refers to a type of water-insoluble contrast agent, which is not suitable for use in medical imaging due to its potential for causing nephrotoxicity and other adverse effects.

**Option B:** This option is incorrect because it refers to a type of oil-based contrast agent, which is not water-soluble and is not suitable for use in medical imaging.

**Option C:** This option is incorrect because it refers to a type of barium-based contrast agent, which is not water-soluble and is not suitable for use in medical imaging.

**Clinical Pearl / High-Yield Fact**
It's essential to note that water-soluble contrast agents are generally safer than oil-based or barium-based agents, but they can still cause adverse effects in certain patients, such as those with iodine allergy or kidney disease.

**Correct Answer: D. Iohexol**