**Core Concept**
Ionic monomers in radiography are a type of contrast agent consisting of a single iodine molecule attached to a monovalent cation. The molecular formula of these agents is typically represented as X-I- where X is the monovalent cation and I is the iodine atom. Ionic monomers exhibit a specific characteristic in solution, where there is a high ratio of iodine atoms to the number of particles present.

**Why the Correct Answer is Right**
The correct answer states that ionic monomers have three iodine atoms per two particles in solution. This is due to the fact that ionic monomers dissociate into two particles (cations and anions) in solution, resulting in a higher concentration of iodine per particle. This characteristic is unique to ionic monomers compared to non-ionic monomers, which do not dissociate in solution and therefore have a lower iodine content. The presence of three iodine atoms per two particles in solution allows ionic monomers to provide a higher attenuation of X-rays, making them effective contrast agents in radiography.

**Why Each Wrong Option is Incorrect**
**Option B:** High osmolar contrast agents are typically ionic in nature, as they dissociate in solution and contribute to the overall osmolality of the agent. Non-ionic contrast agents, on the other hand, are low osmolar and do not dissociate in solution.

**Option C:** Gadolinium-based contrast agents are used primarily in MRI for their paramagnetic properties, which allow them to shorten the T1 relaxation time of water protons. Gadolinium does not cross the blood-brain barrier in significant amounts, making it an effective contrast agent for imaging the brain.

**Option D:** Iohexol is a non-ionic, low osmolar contrast agent, which means it does not dissociate in solution and therefore has a lower iodine content compared to ionic monomers. Iohexol is commonly used in radiography due to its low osmolality and high iodine content.

**Clinical Pearl / High-Yield Fact**
When selecting a contrast agent for radiography, it is essential to consider the type of imaging being performed, the desired level of osmolality, and the patient's renal function. Ionic monomers are often used for high-contrast applications, such as abdominal and pelvic imaging, while non-ionic agents like iohexol are better suited for vascular and cerebral imaging.

**✓ Correct Answer: A. Ionic monomers have three iodine atoms per two particles in solution**