This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Fundamentals in Radiology, Subject name: Radiology
Which of the following contrast agents is preferred in a patient with decreased renal function to avoid contrast nephropathy - 
Select and explain. 
A. Acetylcystine, B. Fenoldapam, C. Mannitol, D. Low osmolar contrast
D: Low osmolar contrast
Answer: D: Low osmolar contrast 
Explanation: Low osmolar contrast agents are agents of choice in patients with pre exiting renal insufficiency Low osmolar contrast agents are ionic dimers and non ionic monomers example for ionic dimer : ioxoglate example for non ionic monomer : iohexol