This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Peritoneal Disorders - Peritonitis, Subject name: Surgery
Most common organ involved in retroperitoneal fibrosis is? 
Select and explain. 
A. Aorta, B. Ureter, C. Inferior vena cava, D. Sympathetic nerve plexus
B: Ureter
Answer: B: Ureter 
Explanation: Ans. (b) UreterRef.: Campbell 10th ed./ 1108-1112* Ureters are involved most commonly.* MC site is lower third.* Obstructive uropathy is the earliest and most common specific symptom.* 70% cases are primary or idiopathic (Ormond's disease)* Investigation of choice is CT scan.* In patients with renal compromise, MRI is the investigation of choice.