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
[{'content': '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: \nSelect and explain. \nA. Emphysematous pyelonephritis, B. Pyonephrosis, C. Renal abscess, D. Acute pyelonephritis', 'role': 'user'}, {'content': 'Answer: C \nExplanation: 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.', 'role': 'assistant'}]