This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Surgery
Patient presenting with marked loin pain, tenderness and pyrexia shows nonspecific findings on IVP but on US reveals a heterogeneous mass with posterior acoustic enhancement and central necrosis and internal debris. CT shows marginal enhancement with air densities within. The likely diagnosis is: 
Select and explain. 
A. Emphysematous pyelonephritis, B. Pyonephrosis, C. Renal abscess, D. Acute pyelonephritis
C: Renal abscess
Answer: C: Renal abscess 
Explanation: Renal abscess Presence of air, rim enhancement on CT and posterio acoustic enhancement in a focal mass with acute clinical history is suggestive of an abscess.